Heat setting optical films

ABSTRACT

A method of making an optical film includes providing a film, substantially uniaxially orienting the film, and heat setting the oriented film. The film includes a polymeric material capable of developing birefringence.

TECHNICAL FIELD

The present disclosure relates to heat setting processes for makingoptical films, as well as films made by these processes.

BACKGROUND

Typically, an optical film that functions as a linear polarizer includesan in-plane block state (“x” direction) and in-plane pass state (“y”direction) for light normally incident to the plane of the film. Thus,light normally incident with a linear polarization state aligned withthe x direction is maximally blocked (i.e. minimally transmitted) andlight normally incident with a linear polarization state aligned withthe y direction is minimally blocked (i.e. maximally transmitted). Lightincident off-normal has intermediate levels of transmittance as afunction of its alignment relative to the film. The axis normal to theplane of the film is referred to as the “z” direction.

SUMMARY

The present disclosure describes a method of making an optical film thatincludes providing a film, substantially uniaxially orienting the film,and heat setting the oriented film. The film includes a polymericmaterial capable of developing birefringence.

The details of one or more embodiments of the present disclosure are setforth in the accompanying drawings and the description below. Otherfeatures, objects, and advantages of the present disclosure will beapparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plot of principal refractive index trends for PEN;

FIG. 2 is a schematic top view of a prior art tenter apparatus used tostretch film;

FIG. 3 is a perspective view of a portion of film in the prior artprocess depicted in FIG. 2 both before and after the stretching process;

FIG. 4 is a plot of asymmetric index growth upon heat setting in PEN;

FIG. 5 is a plot of principal refractive index trends for CoPEN;

FIG. 6 is a plot of principal refractive index trends for CoPEN;

FIG. 7 is a schematic illustration of a prior art batch process fordrawing a multilayer optical film showing the film both before and afterthe stretch;

FIG. 8 is a schematic illustration of the stretching process accordingto one embodiment of the present disclosure;

FIG. 9 is a block diagram showing process steps according to anembodiment of the present disclosure;

FIG. 10 is a schematic top view of a portion of a stretching apparatus;

FIG. 11 is a top view of a portion of the apparatus of FIG. 10;

FIG. 12 illustrates an end view of an arrangement of gripping membersthat may be used in the apparatus of FIG. 10;

FIG. 13 is a schematic illustration of a portion of the tracksillustrating one embodiment of a stretching apparatus;

FIG. 14 is a schematic illustration of one embodiment of adjustabletracks for a primary stretching region of a stretching apparatus;

FIG. 15 is a schematic side cross-sectional view of one embodiment oftracks and a track shape control unit for a stretching apparatus;

FIG. 16 is a schematic view of a portion of the track and track shapecontrol unit of one embodiment of FIG. 10;

FIG. 17 is a schematic view of another portion of the track and trackshape control unit of one embodiment of FIG. 10;

FIG. 18 is a schematic illustration of another embodiment of tracks fora primary stretching region of a stretching apparatus;

FIG. 19 is a graph of examples of suitable boundary trajectories for aprimary stretching region of a stretching apparatus;

FIG. 20 is a graph of examples of suitable boundary trajectories for aprimary stretching region of a stretching apparatus illustrating the useof different stretching regions with different parabolic configurations;

FIG. 21 is a graph of examples of suitable boundary trajectories for aprimary stretching region of a stretching apparatus including boundarytrajectories that are linear approximations to suitable parabolic orsubstantially parabolic boundary trajectories;

FIG. 22 is a schematic illustration of one embodiment of a take-awaysystem for a stretching apparatus according to the present disclosure;

FIG. 23 is a schematic illustration of another embodiment of a take-awaysystem for a stretching apparatus;

FIG. 24 is a schematic illustration of a third embodiment of a take-awaysystem for a stretching apparatus;

FIG. 25 is a schematic illustration of a fourth embodiment of atake-away system for a stretching apparatus;

FIG. 26 is a schematic illustration of a fifth embodiment of a take-awaysystem for a stretching apparatus;

FIG. 27 is a schematic illustration of one embodiment of a take-awaysystem, for using in, for example, a conventional stretching apparatussuch as that illustrated in FIG. 2; and

FIG. 28 is a plot of refractive index trends with composition.

While the above-identified drawing figures set forth several exemplaryembodiments of the disclosure, other embodiments are also contemplated.This disclosure presents illustrative embodiments of the presentinvention by way of representation and not limitation. Numerous othermodifications and embodiments can be devised by those skilled in the artwhich fall within the scope and spirit of the principles of the presentdisclosure. The drawing figures are not drawn to scale.

Moreover, while embodiments and components are referred to by thedesignations “first,” “second,” “third,” etc., it is to be understoodthat these descriptions are bestowed for convenience of reference and donot imply an order of preference. The designations are presented merelyto distinguish between different embodiments for purposes of clarity.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numbers setforth are approximations that can vary depending upon the desiredproperties using the teachings disclosed herein.

DETAILED DESCRIPTION

Many optical films used in polarizer or polarizing film applicationssuffer from an asymmetry, e.g. a refractive index difference, between“y” and “z” principal directions. For example, off-axis color, that is,color variations in the pass state as a function of off-normal angle ofincidence, can be amplified or created by a mismatch between the y and zrefractive indices, ny and nz, respectively. (Here, nx is the refractiveindex for light polarized along the x direction.).

An illustration of the space of typical refractive index sets (nx, ny,nz), for a film or a layer of film consisting of 2,6 polyethylenenapthalate (PEN), is provided in FIG. 1. The development of therefractive index set as a function of draw ratio at an illustrativestretch temperature of 130° C. is exemplified by the data with the solidmarkings. The films were stretched in a laboratory scale stretchingdevice from initially unoriented cast samples. The samples werestretched in one in-plane direction while constraining the film in theother in-plane direction at selected gripping points along the edge,with a nominal initial rate of 20%/sec using a stretch profile thatincreased the nominal draw ratio linearly in time. The true final drawratio was measured using fiducial line markings marked upon the sampleacross the location where the refractive indices were measured. Forillustration purposes, the refractive index was measured at 632.8 nmusing a Metricon Prism Coupler (available from Metricon, Picataway NJ),the red light source provided by He—Ne laser.

FIG. 2 illustrates a conventional tenter stretching process 10 thatstretches film 12 transversely to the direction of film travel 14. Film12 may be continuously fed or introduced as a non-continuous portion offilm 12. The film travel direction is referred to as the machinedirection (MD), and the stretch direction is referred to as thetransverse or tenter direction (TD). The film 12 is gripped at bothedges 16 by some gripping apparatus, typically an arrangement of tenterclips (not shown in FIG. 2). The tenter clips can be connected to tenterchains that ride along linearly diverging tenter tracks or rails. Thisarrangement propels the film forward in a machine direction 14 of filmtravel and stretches the film 12 in the transverse direction. Thus aninitial, unoriented portion 18 in the film may be stretched into afinal, oriented portion 20 in one example. As shown in FIG. 3, theunoriented portion 18 of the film shown in FIG. 2 may have dimensions T(thickness), W (width) and L (length). After the film is stretched by afactor of λ, the dimensions of that portion of film have changed tothose shown on portion 20.

While the data of FIG. 1 derive from a batch stretching device withdiscontinuous edge grips both along the x and y directions, these dataexemplify a typical film stretched in the conventional tenter process ofFIG. 2: one-directional stretching, in which the film 12 is stretched ina first in-plane principal direction (x) while the second in-planedirection (y) is maintained at constant or nearly constant draw ratio,e.g., the y direction draw ratio is nearly unity. In one embodiment, thecontinuous film is fed at a constant rate into a transversely stretchingdevice and exits the device at the same constant rate.

FIG. 1 demonstrates the asymmetry in the refractive indices, e.g. thedifferences in the ny and nz, that develops due to the asymmetrictreatment of the stretch on the material in the y and z directions. Inthis illustrated case, the draw ratio remains nearly constant in y whilethe draw ratio in z decreases with increasing draw ratio in x (e.g. asrequired by near volume conservation as weakly adjusted due todensification with crystallization).

When the film is heat set following the stretching process, theasymmetry is further increased. This situation is exemplified by theopen markers in FIG. 1. In this case, the film was heat set for 2minutes at 175° C.

FIG. 1 shows that the nx and ny values tend to increase under these heatsetting conditions, while the nz values drop, for samples stretchedabove a critical draw ratio level for the given stretch temperature andrate (or equivalently, for a given level of initial refractive indexdevelopment).

FIG. 4 further illustrates how heat setting increases the differencesbetween the ny and nz refractive indices. The square markers indicatethe index difference after stretch without heat setting whereas thetriangular markers indicate the index difference after the stretchingand heat setting. The offset increase in asymmetry is nearly constant,with a small decreasing slope with increasing x draw ratio, as depictedby the diamond markers. Therefore, when y/z “asymmetric” films, e.g.films with significant differences in ny and nz immediately followingstretch, such as those stretched in a conventional tenter (FIGS. 2-3),are heat set following the stretching step, the asymmetry in refractiveindex increases. Such a difference in the refractive indices in the y &z directions can lead to an undesirable color effect in someapplications.

The general trends shown in FIGS. 1 and 4 are applicable to a variety ofpolyesters. Of particular interest are PEN, polyethylene terephthalate(PET) and copolymers of intermediate composition. FIG. 5 illustrates thecase of 85/15 mole percent mixture of PEN-like and PET-like moieties inthe co-polymer, a so-called “85/15 coPEN.” The term “PEN-like” moietyincludes block copolymers of PEN. The term “PET-like” moiety includesblock copolymers of PET. The methods of stretch are nearly identical tothose of the PEN in FIG. 1, except the stretch temperature was set at120° C.

As shown in FIG. 5, in some embodiments, heat setting allows one toachieve a given refractive index at a different draw ratio than withoutheat setting. For example, if one desires an nx of about 1.8, one couldeither use a 4.5 draw ratio or use a lower 2.5 draw ratio and then heatset the film; both processes lead to nx equal to about 1.8. As anotherexample, if one desires an nz of about 1.54, one could either use a 4.25draw ratio or use a lower 2.5 draw ratio and then heat set the film;both processes lead to nz equal to about 1.54.

Under the conditions of FIG. 5, the effective point of significantstrain-induced crystallization is at an x draw ratio of about 2.2. FIG.6 illustrates the sharpness of this transition, with materials stretchedto an x draw ratio of about 2.1 exhibiting relaxation to isotropy afterheat setting. Below an x draw ratio of about 2.1, nx=ny=nz after heatsetting. Above an x draw ratio of about 2.2, index development leads toa result of nx>ny>nz after heat setting. This point will shift indifferent examples, depending on factors such as the selected materialsand processing conditions.

Use of a parabolic tenter (discussed with reference to FIG. 10, below)can lead to uniaxial stretching at relatively high draw ratios in someembodiments. Use of other machines and processes can lead to uniaxialstretching at lower draw ratios, but the resulting film may lack thedesired level of refractive index development. Heat setting can be usedto achieve the desired level of index development, as discussed withreference to FIGS. 5 and 6. In some cases, lower draw ratios are usedwith certain films, such as films including microstructures, becausehigher draw ratios could damage the microstructures. In these and othercases, heat setting can also be used to promote refractive indexdevelopment in the stretched films.

Commonly owned U.S. Pat. Nos. 6,939,499; 6,916,440; 6,949,212; and6,936,209; incorporated herein by reference, describe continuousprocesses for processing optical films, such as multilayer opticalfilms. In such processes, the optical film is oriented by stretchingalong a first in-plane axis of the film (x direction) while allowingcontraction of the film in the second in-plane axis (y or machinedirection (MD)) and in the thickness (z or normal direction (ND)) of thefilm. The stretching is achieved by grasping edge portions of the filmand moving the edge portions of the film along predetermined paths thatdiverge to create substantially similar proportional dimensional changesin the second in-plane axis of the film (y) and in the thicknessdirection (z) of the film.

In exemplary embodiments, in contrast to the heat set behavior ofconventional one-direction stretched materials, which have significantdifferences in ny and nz immediately following stretching, the heatsetting of substantially uniaxially stretched films, in whichcontraction is allowed in the y and z directions to minimize differencesin ny and nz, has a completely different effect. Heat setting followinga substantially uniaxial stretching process maintains or decreases anysmall existing refractive index asymmetry of these films. Thus, wherethe refractive indices in the y & z directions become more equal, fewerproblems with undesirable color effects arise.

The heat setting procedures described below may be applied following anyprocess that provides substantially uniaxial stretching of an opticalfilm such as, for example, a multilayer optical film (MOF). The heatsetting procedures described in this disclosure are particularly usefulfor substantially uniaxially stretched films including one or morepolyester layers.

FIG. 7 illustrates a batch technique 22 for substantially uniaxiallystretching an optical film such as, for example, a multilayer opticalfilm, suitable for use as a component in an optical body such as apolarizer. The film 24 is stretched in the direction of the arrows 26,and the central portion 28 necks down so that two edges 30 of the film24′ are no longer parallel after the stretching process. The centralportion 28 of the film 24′ provides the most useful optical propertiesbecause it is far enough removed from the shear boundary conditions toexperience low levels of shear aberrations such as caliper variation.

While the batch process described in FIG. 7 may in some cases providesuitable film properties, the substantially uniaxial stretchingprocesses described in commonly owned U.S. Pat. Nos. 6,939,499;6,916,440; 6,949,212; and 6,936,209; all incorporated herein byreference, are particularly suitable in some embodiments.

In general, uniaxial orientation of a birefringent polymer provides anoptical film (or layer of a film) in which the index of refraction intwo of three orthogonal directions is substantially the same (forexample, the width (W) and thickness (T) direction of a film, asillustrated in FIG. 8). The index of refraction in the third direction(for example, along the length (L) direction of the film) is differentfrom the indices of refraction in the other two directions. Typically,perfect uniaxial orientation is not required and some degree ofdeviation from the optimal conditions can be allowed depending on avariety of factors including the end-use application of the opticalfilm. Moreover, it should be understood that while the presentdisclosure refers to three “orthogonal directions,” the correspondingdirections may not be exactly orthogonal due to non-uniformities in thefilm.

In general, the substantially uniaxial stretching process includesstretching a film that can be described with reference to three mutuallyorthogonal axes corresponding to the machine direction (MD), thetransverse direction (TD), and the normal direction (ND). These axescorrespond to the width, length, and thickness of the film, asillustrated in FIG. 8. A substantially uniaxial stretching processstretches film 32 from an initial configuration 34 to a finalconfiguration 36. The machine direction (MD) is the general directionalong which the film travels through a stretching device, for example,the apparatus as illustrated in FIG. 10. The transverse direction (TD)is the second axis within the plane of the film and is orthogonal to themachine direction. The normal direction (ND) is orthogonal to both MDand TD and corresponds generally to the thickness dimension of thepolymer film.

FIG. 9 is a block diagram of a typical substantially uniaxial stretchingprocess 38 as described in this disclosure. In step 40, a film issupplied or provided to a stretching apparatus. The process optionallyincludes a preconditioning step 42. The film is stretched in step 44.The film is post-conditioned in step 46. The film is removed from thestretching apparatus in step 48.

FIG. 10 illustrates one embodiment of a substantially uniaxialstretching process and an apparatus 50 for achieving the substantiallyuniaxial stretch. This process may be used with the heat settingprocedures described in this disclosure. It will be recognized that theprocess illustrated by FIG. 9 can be accomplished using one or moreapparatuses apart from a stretching apparatus (which at a minimumperforms step 44 of FIG. 9). These one or more additional apparatusesmay perform one or more of the functions (for example, functionsrepresented by steps 40, 42, 46 and 48 illustrated in FIG. 9.

In the illustrated embodiment of FIG. 10, the apparatus 50 includes aregion 52 where the film 32 is introduced into the stretching apparatus50. The film 32 can be provided by any desirable method. For example,the film 32 can be produced in a roll or other form and then provided tostretching apparatus 50. As another example, the stretching apparatus 50can be configured to receive the film 32 from an extruder (if, forexample, the film 32 is generated by extrusion and ready for stretchingafter extrusion) or a coater (if, for example, the film 32 is generatedby coating or is ready for stretching after receiving one or more coatedlayers) or a laminator (if, for example the film 32 is generated bylamination or is ready for stretching after receiving one or morelaminated layers).

Generally, the film 32 is presented in region 52 to one or more grippingmembers that hold opposing edges of the film and convey the film alongopposing tracks 54 defining conveyance paths. The gripping members (seeFIG. 12, for example) typically hold the film 32 at or near the edges ofthe film 32. The portions of the film 32 held by the gripping membersare often unsuitable for use after stretching, so the position of thegripping members is typically selected to provide sufficient grip on thefilm 32 to permit stretching while controlling the amount of wastematerial generated by the process.

Gripping members, such as clips, can be directed along the track 54 by,for example, rollers 56 rotating a chain along the track 54 with thegripping members coupled to the chain. The rollers 56 are connected to adriver mechanism that controls the speed and direction of the film 32 asit is conveyed through the stretching apparatus 50. Rollers 56 can alsobe used to rotate and control the speed of belt-type gripping members.The belts and rollers 56 optionally include interlocking teeth to reduceor prevent slippage between the belt and roller 56.

The apparatus 50 optionally includes a preconditioning region 58 that inone embodiment is enclosed by an oven 60 or other apparatus orarrangement to heat the film 32 in preparation for stretching. Thepreconditioning region 58 can include a preheating zone 62, a heat soakzone 64, or both. In at least some embodiments, there may be a smallamount of film stretching that occurs in order to set the contactbetween the gripping members and the film, as illustrated by theboundary trajectory of FIG. 13. In at least some instances, there maynot actually be any stretching but the increase in separation betweenthe opposing tracks may account, at least in part, for thermal expansionof the film 32 as the film 32 is heated.

The film 32 is stretched in the primary stretching region 66. Typically,within the primary stretching region 66 the film zone undergoingstretching 68 is heated or maintained in a heated environment above theglass transition of the polymer(s) of the film 68. For polyesters, thetemperature range is typically between about 80° C. and about 160° C.Examples of suitable heating elements include convective and radiativeheating elements, although other heating elements can also be used. Insome embodiments, the heating elements used to heat the film 32 can becontrolled individually or in groups to provide a variable amount ofheat. Such control can be maintained by a variety of processes includingthose that allow for variability in the temperature of the heatingelements or in the direction or speed of air directed from the heatingelement to the film 68. The control of the heating elements can be used,if desired, to variably heat regions of the film 68 to improve orotherwise alter uniformity of stretching across the film 68. Forexample, areas of the film 68 that do not stretch as much as other areasunder uniform heating can be heated more to allow easier stretching.

Within the primary stretching region 66, the gripping members followgenerally diverging tracks 54 to stretch the polymer film 68 by adesired amount. The tracks 54 in the primary stretching region 66 and inother regions of the apparatus 50 can be formed using a variety ofstructures and materials. Outside of the primary stretching region 66,the tracks 54 are typically substantially linear. The opposing lineartracks 54 can be parallel or can be arranged to be converging ordiverging. Within the primary stretching region 66, the tracks 54 aregenerally diverging and are generally curvilinear. In some exemplaryembodiments, the generally curvilinear shapes of the tracks may beapproximated using linear track segments.

In one example, the apparatus 50 typically includes a post-conditioningregion 70. For example, the film 32 may be heat set in zone 72 andquenched in zone 74. The film 32 is quenched when all components reach atemperature level below their glass transition temperatures. In someother embodiments, quenching is performed outside the stretchingapparatus 50.

In the embodiment illustrated in FIG. 10, a takeaway system is used toremove the film 32 from the primary stretching region 66. In theillustrated embodiment, this takeaway system is independent of (i.e.,not directly connected to) the tracks 54 upon which the film 32 wasconveyed through the primary stretching region 66.

For the purposes of this disclosure, the term heat set refers to aheating protocol in which the film 32 is heated following orientation toenhance film properties such as, for example, crystal growth,dimensional stability, and/or overall optical performance. The heatsetting is a function of both temperature and time, and factors must beconsidered such as, for example, commercially useful line speed and heattransfer properties of the film, as well as the optical clarity of thefinal product. In an exemplary embodiment, the heat setting processinvolves heating the film 32 to above the glass transition temperature(Tg) of at least one polymeric component thereof, and preferably abovethe Tg of all polymeric components thereof. Exemplary polymericmaterials include PEN, PET, coPENS, polypropylene and syndiotacticpolystyrene. In one embodiment of the heat setting process, the film 32is heated above the stretch temperature of the film 32, although this isnot required. In another embodiment, in the heat setting process thefilm 32 is heated to a temperature between the Tg and the melting pointof the film 32.

In general, there is an optimal temperature for the rate ofcrystallization that results from a balance of the kinetic andthermodynamics of the system. This temperature is useful whenminimization of the heat set time is a primary consideration. A typicalstarting point for tuning the conditions to find the best balancebetween the various product and process considerations is about halfwaybetween the Tg and the melting point of the film 32. For example, theglass transition temperatures for PET and PEN are approximately 80° C.and 120° C., respectively, under dry conditions. The glass transitiontemperatures of copolymers of intermediate compositions of PET and PEN(so-called “coPENs”) are intermediate between those of the homopolymers.The melting points cover a range of temperatures due to the range ofimperfections in the physical crystals due to their size andconstraints. A rough estimate for the melting points of PET and PEN isabout 260° C. for PET and about 270° C. for PEN. The melting points ofthe so-called coPENs are typically less than those of the homopolymersand can be measured approximately, for example by Differential Scanningcalorimetry (DSC).

Thus, the starting point range for heat setting in PET and PEN is, forexample, between about 170 and 195° C. Actual process setpoints dependon residence times and heat transfer within a given process. Residencetimes may range from about 1 second to about 10 minutes and depend notonly on process conditions but also the desired final effect, forexample, the amount of crystallinity, the increase in delaminationresistance, and optimization of haze given other properties. Minimizingthe residence time is often useful for considerations such as minimizingequipment size. Higher temperatures may reduce the required time toattain a certain level of crystallinity. However, higher temperaturesalso may cause melting of imperfect crystalline structures that may thenre-form into larger structures. This may produce unwanted haze for someapplications.

In one embodiment, the portions of the film that were held by thegripping members through the primary stretching region 66 are removed.To maintain a substantially uniaxial stretch throughout substantiallyall of the stretch history (as shown in FIG. 10), at the end of thetransverse stretch, the rapidly diverging edge portions 76 arepreferably severed from the stretched film 68 at a slitting point 78. Acut can be made at 78 and flash or unusable portions 76 can bediscarded.

In one embodiment, the process also includes a removal region 80.Optionally a roller 82 is used to advance the film, but this may beeliminated. In one embodiment, the roller 82 is not used as it wouldcontact the final film 84 with the attendant potential to damage thefinal film 84. In one embodiment, another cut 86 is made and unusedportion 88 is discarded.

The removal region 80 may also include an optional isolation zone (notshown in FIG. 10) in which the film temperature is controlled to reduceand/or eliminate undesirable film properties such as bowing. In theisolation zone, the film may be wound onto a roll, but winding is notrequired. Following removal from the optional isolation zone, the filmmay optionally be coated or laminated, or subjected to processing toimpart a surface texture or surface structures.

In one embodiment, direct converting to a finished product takes placeafter take-away. In another embodiment, film 84 leaving the take-awaysystem is typically wound on rolls for later use. In one example, thefilm 84 may be unwound and transferred to an optional second heatingunit (not shown in FIG. 10). In the second heating unit, the film 84 maybe gripped and placed under tension as needed to prevent wrinkling. Thisprocess typically takes place at a temperature below the originalstretch temperature applied in the stretching zone 66. The secondheating unit may simply be an oven where the film 84 may be placed inroll or sheet form to enhance its properties. For example, a second heatsoak procedure may be applied in the second heating zone in which thefilm 84 is heated to a temperature below the Tg of at least one filmcomponent, preferably below the Tg of all film components. Again, thesecond heat soak is typically performed below the initial stretchtemperature applied to the film 84 in the stretching zone 66. The secondheat soak may continue for an extended period such as, for example,hours or days, until the desired film properties such as shrinkageresistance, or creep resistance are achieved. For example, heat soak forPET is typically performed at about 50-75° C. for several hours to days,while heat soak for PEN is typically performed at about 60-115° C. forseveral hours to days. Heat soaking can also be achieved in part undersome post-processing activities. For example, the film 84 may be coatedand dried or cured in an oven with some heat soaking effect.

Following the second heating zone, the film 84 may optionally betransferred to a second quench and/or set zone (not shown in FIG. 10).In the second quench and/or set zone, the film 84 may be placed undertension and/or toed-in along converging rails to control shrinkage andwarping. Following the optional second quench and/or set zone, the filmmay be re-rolled.

FIGS. 11-12 illustrate one embodiment of the gripping members and track.One example of suitable gripping members 90 includes a series of clipsthat sequentially grip the film 32 between opposing surfaces and thentravel around a track 54. The gripping members can nest or ride in agroove or a channel along the track 54. Another example is a belt systemthat holds the film 32 between opposing belts or treads, or a series ofbelts or treads, and directs the film 32 along the track 54. Belts andtreads can, if desired, provide a flexible and continuous, orsemi-continuous, film conveyance mechanism. A variety of opposing,multiple belt methods are described, for example, in U.S. Pat. No.5,517,737 and in European Patent Application Publication No. 0236171 A1(the entire contents of each of which are herein incorporated byreference). The tension of the belts is optionally adjustable to obtaina desired level of gripping.

A belt or clip can be made of any material. For example, a belt can beof composite construction. One example of a suitable belt includes aninner layer made of metal, such as steel, to support high tension and anouter layer of elastomer to provide good gripping. Other belts can alsobe used. In some embodiments, the belt includes discontinuous tread toprovide good gripping.

Other methods of gripping and conveying the film through a stretcher areknown and may be used. In some embodiments, different portions of thestretching apparatus can use different types of gripping members 90.

The gripping members 90 of the embodiment illustrated in FIGS. 11 and 12are a series of tenter clips. These clips can afford overall flexibilityvia segmentation. The discrete clips are typically closely packed andattached to a flexible structure such as a chain. The flexible structurerides along or in channels along the track 54. Strategically placed camsand cam surfaces open and close the tenter clips at desired points. Theclip and chain assembly optionally rides on wheels or bearings or thelike. In one example, the gripping members 90 are tenter clips mountedon top and bottom bearings rolling between two pairs of inner and outerrails. These rails form, at least in part, the track 54.

The edges of the gripping members 90 define a boundary edge for theportion of the film 32 that will be stretched. The motion of thegripping members 90 along the tracks 54 provides a boundary trajectorythat is, at least in part, responsible for the motion and stretching ofthe film 32. Other effects (e.g., downweb tension and take-up devices)may account for other portions of the motion and stretching. Theboundary trajectory is typically more easily identified from the track54 or rail along which the gripping members 90 travel. For example, theeffective edge of the center of the gripping member 90, e.g. a tenterclip, can be aligned to trace the same path as a surface of the track 54or rail. This surface then coincides with the boundary trajectory. Inpractice, the effective edge of the gripping members 90 can be somewhatobscured by slight film slippage from or flow out from under thegripping members 90, but these deviations can be made small.

In addition, for gripping members 90 such as tenter clips the length ofthe edge face can influence the actual boundary trajectory. Smallerclips will in general provide better approximations to the boundarytrajectories and smaller stretching fluctuations. In at least someembodiments, the length of a clip face edge is no more than aboutone-half the total initial distance between the opposing boundarytrajectories or tracks. In a particularly suitable embodiment, thelength of a clip face edge is no more than about ¼ the total initialdistance between the opposing boundary trajectories or tracks.

The two opposing tracks 54 are optionally disposed on two separate orseparable platforms or are otherwise configured to allow the distancebetween the opposing tracks 54 to be adjustable. This can beparticularly useful if different sizes of film 32 are to be stretched bythe apparatus 50 or if there is a desire to vary the stretchingconfiguration in the primary stretching region 66, as discussed below.Separation or variation between the opposing tracks 54 can be performedmanually, mechanically (for example, using a computer or other device tocontrol a driver that can alter the separation distance between thetracks 54), or both.

Since the film 32 is held by two sets of opposing gripping members 90mounted on opposing tracks 54, there are two opposing boundarytrajectories. In at least some embodiments, these trajectories aremirror images about an MD center line of the stretching film 32. Inother embodiments, the opposing tracks 54 are not mirror images. Such anon-mirror image arrangement can be useful in providing a variation (forexample, a gradient or rotation of principal axes) in one or moreoptical or physical properties across the film 32.

FIG. 13 illustrates one embodiment of a supply region 52 followed by apreconditioning region 58 and primary stretching region 66. Within thepreconditioning region 58 (or optionally in the supply region 52) agripping member set zone 92 is provided in which the tracks 54 divergeslightly to set the gripping members 90 (for example, tenter clips) onthe film 32. The film 32 is optionally heated within this zone 92. Inone embodiment, this initial TD stretch is no more than about 5% of thefinal TD stretch and generally less than about 2% of the final TDstretch and often less than about 1% of the final TD stretch. In someembodiments, the zone 92 in which this initial stretch occurs isfollowed by a zone 94 in which the tracks 54 are substantially paralleland the film 32 is heated or maintained at an elevated temperature.

In all regions of the stretching apparatus 50, the tracks 54 can beformed using a series of linear or curvilinear segments that areoptionally coupled together. The tracks 54 can be made using segmentsthat allow two or more (or even all) of the individual regions to beseparated (for example, for maintenance or construction). As analternative or in particular regions or groups of regions, the tracks 54can be formed as a single continuous construction. The tracks 54 caninclude a continuous construction spanning one or more adjacent regions52, 58, 66, 70, 80 of the stretcher 50. The tracks 54 can have anycombination of continuous constructions and individual segments.

In some embodiments, the tracks 54 in the primary stretching region 66are coupled to, but separable from, the tracks 54 of the precedingregions. In some embodiments, the tracks in the succeedingpost-conditioning or removal regions 70, 80 are typically separated fromthe tracks 54 of the primary stretching region 66, as illustrated, forexample, in FIGS. 22-27.

Although the tracks in the primary stretching region 66 are curvilinearin FIG. 10, linear track segments can also be used in some embodiments.In one embodiment, these segments are aligned (by, for example, pivotingindividual linear segments about an axis) with respect to each other toproduce a linear approximation to a desired curvilinear trackconfiguration. Generally, the shorter the linear segments are, thebetter the curvilinear approximation can be made. In some embodiments,the positions of one or more, and preferably all, of the linear segmentsare adjustable (pivotable about an axis) so that the shape of the tracks54 can be adjusted if desired. Adjustment can be manual or theadjustment can be performed mechanically, such as under control of acomputer or other device coupled to a driver. It will be understood thatcurvilinear segments can be used instead of or in addition to linearsegments.

Continuous tracks 54 can also be used through each of the regions 52,58, 66, 70, 80. In particular, a continuous, curvilinear track 54 can beused through the primary stretching region 66. The continuous,curvilinear track 54 typically includes at least one continuous railthat defines the track 54 along which the gripping members 90 run. Inone embodiment, the curvilinear track 54 includes two pairs of inner andouter rails with tenter clips mounted on top and bottom bearings rollingbetween the four rails.

In some embodiments, the continuous track 54 is adjustable. One methodof making an adjustable continuous track 54 includes the use of one ormore track shape control units. These track shape control units arecoupled to a portion of the continuous track 54, such as the continuousrail, and are configured to apply a force to the track 54 as required tobend the track 54. FIG. 14 schematically illustrates one embodiment ofsuch an arrangement with the track shape control units 96 coupled to thetrack 54. Generally, the track shape control units 96 have a range offorces that the track shape control unit 96 can apply, although someembodiments may be limited to control units 96 that are either on oroff.

The track shape control units 96 can typically apply a force toward thecenter of the film 32 or apply a force away from the center of the film32 or, preferably, both. The track shape control units 96 can be coupledto a particular point on the adjustable continuous track 54 or the trackshape control units 96 can be configured so that the track 54 can slidelaterally along the control unit 96 while still maintaining couplingbetween the track 54 and control unit 96. This arrangement canfacilitate a larger range of motion because it allows the track 54 tomore freely adjust as the control units 96 are activated. Generally, thetrack shape control units 96 allow the track 54 to move through a rangeof shapes that deviate from the equilibrium shape of the track 54, forexample, shapes 54 and 54′ of FIG. 14. The equilibrium and the adjustedshapes of the tracks may be linear or curvilinear. Typically, the trackshape control unit 96 and the track 54 can move along a line (or othergeometric shape) of motion 98. When more than one track shape controlunit 96 is used, the track shape control units 96 can have the same orsimilar lines of motion and ranges of motion 98 or the lines and rangesof motions 98 for the individual track shape control units 96 can bedifferent.

In some embodiments, one or more points 100 of the track are fixed. Thefixed points 100 can be anywhere along the track 54 including at or nearthe start (as illustrated in FIG. 14) or end of the primary stretchingregion 66. The fixed points 100 can also be positioned at other pointsalong the track 54 as illustrated in FIG. 18.

One example of a suitable track shape control unit 96 and track 54 isillustrated in FIG. 15. The track 54 in this embodiment includes fourrails 102 with tenter clips (not shown) mounted on bearings (not shown)rolling between the four rails 102. The track shape control unit 96includes a base 104 that is coupled to a driver (not shown), top andbottom inner contact members 106, and top and bottom outer contactmembers 108. The inner and outer contact members 106, 108 are coupled tothe base 104 so that moving the base 104 allows the contact members 106,108 to apply a force to inner and outer surfaces of the rails 102,respectively.

In exemplary embodiments, the inner contact members 106 have a shape,when viewed from above or below, that provides only small areas ofcontact between the contact members 106, 108 and the rails 102, asillustrated in FIGS. 16 and 17 (FIG. 16 shows the rails 102 and innercontact member 106). Examples of such shapes include circular and ovoid,as well as diamond, hexagonal, or other similar shapes where contactbetween the inner contact members 106 and the rails 102 is made at theapex of these shapes. The outer contact members 108 can be similarlyfashioned so that the portion of the outer contact member 108, whenviewed from above or below, comes to a point to make contact with therails 102, as illustrated in FIG. 17 (FIG. 17 shows the rails 102 andthe portion of the outer contact member 108 that makes contact with therails 102). Using such shapes allows the track shape control unit 96 toexert a force, if desired, to modify the track shape while allowing thetrack 54 to slide laterally through the control unit 96 rather thanbeing fixed to the control unit 96. This configuration can also allowthe track 54 to adjust its instantaneous slope within the control unit96. For one or both of these reasons, the track 54 can have a largerrange of shape adjustment. In other embodiments, there can be fewer ormore contact members 106, 108 or there may be only inner or only outercontact members 106, 108.

As further illustrated in FIG. 18, the tracks 54 can be configured toprovide zones 110, 112, 114 within the primary stretching region 66 thathave different stretching characteristics or that can be described bydifferent mathematical equations. In some embodiments, the tracks 54have a shape that defines these different zones 110, 112, 114. In otherembodiments, the tracks 54 can be adjusted, using for example the trackshape control units 96 discussed above, to provide a variety of shapes116, 118 beyond simple, monofunctional arrangements. This can beadvantageous because it allows different portions of the primarystretching region 66 to accomplish desired functions. For example, aninitial stretching zone may have a particular shape (for example, asuper-uniaxial shape with U>1 and F>1 as described below) followed byone or more later zones with different shapes (for example, a uniaxialshape). Optionally, intermediate zones can be provided that transitionfrom one shape to another. In some embodiments, the individual zones110, 112, 114 can be separated or defined by points 100 of the track 54that are fixed.

In some embodiments, the track 54 has a non-uniform cross-sectionalshape along the length of the track 54 to facilitate bending and shapingof the track 54. For example, one or more rails 102 used in the track 54can have different cross-sectional shapes. As an example, in thefour-rail construction described above, each of the rails 102, or asubset of the rails 102, has a varied cross-section along the length ofthe track 54. The cross-section can be varied by, for example, alteringeither the height or thickness of the track 54 (or a component of thetrack 54 such as one or more continuous rails 102) or both. As anexample, in one embodiment the thickness of the track 54 or one or morerails 102 in the track 54 decreases or increases along the length of thetrack 54 in the machine direction. These variations can be used tosupport a particular track shape or a variation in track shapeadjustability. For example, as described above, the track 54 may haveseveral different zones 110, 112, 114, each zone 110, 112, 114 having adifferent track shape 54. The cross-sectional variation of the track 54or component of the track 54 can vary within each zone 110, 112, 114 toachieve or facilitate a particular rail 102 shape and can vary betweenzones 110, 112, 114. As an example, a zone 112 with a relatively thickcross-sectional shape can be disposed between two other zones 110, 114to isolate or provide a transitional space between the two zones 110,114.

As an example of variation in track 54 or rail 102 cross-section, thearclength, s, can be used to represent a position along the track 54 inthe design of the thickness profile of a track 54 or portion of a track,such as a rail 102. The arclength, s, at the start of stretch is definedas zero and at the other end of the stretch is defined as L withcorresponding thicknesses at the beginning and end of stretch beingdesignated as h(0) and h(L), respectively. The track 54 or trackcomponent (e.g., rail 102) in this particular embodiment has a taperover a portion of the beam from L′ to L″ between s=0 and s=L such thatthe thickness h(L′) at position L′ is greater than the thickness h(L″)at position L″. In this manner, either L′ or L″ may be at the higherarclength coordinate (i.e., L′>L″ or L′<L″). One example of a usefulthickness profile is a taper given by the function for thickness, h(s),as a function of arclength s over the rail 102 from L′ to L″, providedby the equation:

h(s)=(h(L′)−h(L″))(1−(s−L′)/(L″−L′))^(α) +h(L″)

where α is the positive rate of taper resulting in decreasing thicknessfrom L′ to L″.

When L′ is less than L″, this results in a decreasing thickness witharclength. When L′ is greater than L″ this results in an increasingthickness with arclength. The track 54 can optionally be apportionedinto sections, each with its own local L′, L″ and rate of taper. Themaximum thickness of the track 54 or track component, such as a rail,depends on the amount of flexibility desired at that point on the track54. Using beam theory as applied to a track or rail, it can be shownthat in the case of a straight beam with a taper, a value for a of onethird provides a beam that bends parabolically in response to a load atone end. When the beam begins in a curved equilibrium configuration oris loaded by several control points, other tapers may be more desirable.For transformation across a variety of other shapes, it may be useful tohave both increasing and decreasing thickness within a given track 54 ortrack component, or numerically calculated forms of the taper over anyof these sections. The minimum thickness at any point along the track 54or track component depends on the amount of required strength of thetrack 54 to support the stretching forces. The maximum thickness can bea function of the level of needed flexibility. It is typicallybeneficial to maintain the level of track adjustment within the elasticrange of the track 54 or track component, e.g. to avoid the permanentyielding of the track 54 or track component and loss of repeatableadjustment capability.

The paths defined by the opposing tracks 54 affect the stretching of thefilm 32 in the MD, TD, and ND directions. The stretching transformationcan be described as a set of draw ratios: the machine direction drawratio (MDDR), the transverse direction draw ratio (TDDR), and the normaldirection draw ratio (NDDR). When determined with respect to the film32, the particular draw ratio is generally defined as the ratio of thecurrent size (for example, length, width, or thickness) of the film 32in a desired direction (for example, TD, MD, or ND) and the initial size(for example, length, width, or thickness) of the film 32 in that samedirection. Although these draw ratios can be determined by observationof the polymer film 32 as stretched, unless otherwise indicated,reference to MDDR, TDDR, and NDDR refers to the draw ratio determined bya track 54 used to stretch the polymer film 32.

At any given point in the stretching process, TDDR corresponds to aratio of the current separation distance of the boundary trajectories,L, and the initial separation distance of the boundary trajectories, L₀,at the start of the stretch. In other words, TDDR=L/L₀. In someinstances (as in FIGS. 2 and 9, for example), TDDR is represented by thesymbol X. At any given point in the stretching process, MDDR is thecosine of the divergence angle, θ, the positive included angle betweenMD and the instantaneous tangent of the boundary trajectory, e.g. track54 or rail 102. It follows that cot(0) is equal to the instantaneousslope (i.e., first derivative) of the track 54 at that point. Upondetermination of TDDR and MDDR, NDDR=1/[(TDDR)(MDDR)] provided that thedensity of the polymer film is constant during the stretching process.If, however, the density of the film changes by a factor of ρ_(f), whereρ_(f)=ρ₀/ρ with ρ being the density at the present point in thestretching process and ρ₀ being the initial density at the start of thestretch, then NDDR=ρ_(f)/[(TDDR)(MDDR)] as expected. A change in densityof the material can occur for a variety of reasons including, forexample, due to a phase change, such as crystallization or partialcrystallization, caused by stretching or other processing conditions.

Perfect uniaxial stretching conditions, with an increase in dimension inthe transverse direction, result in TDDR, MDDR, and NDDR of λ,(λ)^(−1/2), and (λ)^(−1/2), respectively, as illustrated in FIG. 8(assuming constant density of the material). In other words, assuminguniform density during the stretch, a uniaxially oriented film is one inwhich MDDR=(TDDR)^(−1/2) throughout the stretch. A useful measure of theextent of uniaxial character, U, can be defined as:

$U = \frac{\frac{1}{MDDR} - 1}{{TDDR}^{1/2} - 1}$

For a perfect uniaxial stretch, U is one throughout the stretch. When Uis less than one, the stretching condition is considered “subuniaxial”.When U is greater than one, the stretching condition is considered“super-uniaxial”. In a conventional tenter, the polymer film 12 isstretched linearly along edges 16, as illustrated in FIG. 2, to stretcha region 18 of the film to a stretched region 20. In this example, thedivergence angle is relatively small (e.g., about 3° or less), MDDR isapproximately 1 and U is approximately zero. If the film 12 is biaxiallystretched so that MDDR is greater than unity, U becomes negative. Insome embodiments, U can have a value greater than one. States of Ugreater than unity represent various levels of over-relaxing. Theseover-relaxed states produce an MD compression from the boundary edge. Ifthe level of MD compression is sufficient for the geometry and materialstiffness, the film will buckle or wrinkle.

As expected, U can be corrected for changes in density to give U_(f)according to the following formula:

$U_{f} = \frac{\frac{1}{MDDR} - 1}{\left( \frac{TDDR}{\rho_{f}} \right)^{1/2} - 1}$

Preferably, the film is stretched in plane (i.e., the boundarytrajectories and tracks are coplanar) such as shown in FIG. 10, althoughnon-coplanar stretching trajectories are also acceptable. The design ofin-plane boundary trajectories is simplified because the in-planeconstraint reduces the number of variables. The result for a perfectuniaxial orientation is a pair of mirror symmetric, in-plane, parabolictrajectories diverging away from the in-plane MD centerline. Theparabola may be portrayed by first defining TD as the “x” direction andMD as the “y” direction. The MD centerline between the opposing boundingparabolas may be taken as the y coordinate axis. The coordinate originmay be chosen to be the beginning of the primary stretching region 66and corresponds to the initial centerpoint of the central trace betweenthe parabolic trajectories. The left and right bounding parabolas arechosen to start (y=0) at minus and plus x₀, respectively. The rightbounding parabolic trajectory, for positive y values, that embodies thisembodiment of the present disclosure is:

x/x ₀=(¼)(y/x ₀)²+1

The left bounding parabolic trajectory is obtained by multiplying theleft-hand side of the above equation by minus unity. In the discussionbelow, descriptions of and methods for determining the right boundedtrajectory are presented. A left bounded trajectory can then be obtainedby taking a mirror image of the right bounded trajectory over thecenterline of the film.

For sub-uniaxial stretches, the final extent of truly uniaxial charactercan be used to estimate the level of refractive index matching betweenthe y (MD) and z (ND) directions by the equation:

Δn _(yz) =Δn _(yz)(U=0)×(1−U)

where Δn_(yz) is the difference between the refractive index in the MDdirection (i.e., y-direction) and the ND direction (i.e., z-direction)for a value U and Δn_(yz)(U=0) is that refractive index difference in afilm stretched identically except that MDDR is held at unity throughoutthe stretch. This relationship has been found to be reasonablypredictive for polyester systems (including PEN, PET, and copolymers ofPEN or PET) used in a variety of optical films. In these polyestersystems, Δn_(yz)(U=0) is typically about one-half or more of thedifference Δn_(xy)(U=0), which is the refractive difference between thetwo in-plane directions MD (y-axis) and TD (x-axis). Typical values forΔn_(xy)(U=0) range up to about 0.26 at 633 nm. Typical values for4n_(yz)(U=0) range up to about 0.15 at 633 nm. For example, a 90/10coPEN, i.e. a copolyester comprising about 90% PEN-like repeat units and10% PET-like repeat units, has a typical value at high extension ofabout 0.14 at 633 nm. Films comprising this 90/10 coPEN with values of Uof 0.75, 0.88 and 0.97 as measured by actual film draw ratios withcorresponding values of Δn_(yz) of 0.02, 0.01 and 0.003 at 633 nm havebeen made according to the methods described herein.

A coplanar parabolic trajectory can provide uniaxial orientation underideal conditions. However, other factors can affect the ability toachieve uniaxial orientation including, for example, non-uniformthickness of the polymer film, non-uniform heating of the polymer filmduring stretching, and the application of additional tension (forexample, machine direction tension) from, for example, down-web regionsof the apparatus. In addition, in many instances it is not necessary toachieve perfect uniaxial orientation. Instead, a minimum or threshold Uvalue or an average U value that is maintained throughout the stretch orduring a particular portion of the stretch can be defined. For example,an acceptable minimum/threshold or average U value can be 0.2, 0.5, 0.7,0.75, 0.8, 0.85, 0.9, or 0.95, as desired, or as needed for a particularapplication. Generally, any minimum/threshold or average U value that ismore than 0 is suitable.

As an example of acceptable nearly uniaxial applications, the off-anglecharacteristics of reflective polarizers used in liquid crystallinedisplay applications is strongly impacted by the difference in the MDand ND indices of refraction when TD is the principal stretch direction.An index difference in MD and ND of 0.08 is acceptable in someapplications. A difference of 0.04 is acceptable in others. In morestringent applications, a difference of 0.02 or less is preferred. Forexample, the extent of uniaxial character of 0.85 is sufficient in manycases to provide an index of refraction difference between the MD and NDdirections in polyester systems containing polyethylene naphthalate(PEN) or copolymers of PEN of 0.02 or less at 633 nm forsingle-direction transversely stretched films. For some polyestersystems, such as polyethylene terephthalate (PET), a lower U value of0.80 or even 0.75 may be acceptable because of lower intrinsicdifferences in refractive indices in non-substantially uniaxiallystretched films.

Oriented optical films made by methods of the present disclosure includereflective polarizers such as multilayer reflective polarizers anddiffusely reflective polarizers. Descriptions of the latter can be foundin commonly owned U.S. Provisional Application Ser. No. 60/668,944,filed Apr. 6, 2005, and U.S. application Ser. No. 11/398,276, filed Apr.5, 2006, both entitled, “Diffuse Reflective Polarizing Films withOrientable Polymer Blends,” and in U.S. Pat. Nos. 5,825,543, 6,057,961,6,590,705, and 6,057,961, incorporated herein by reference. Suchdiffusely reflective polarizers include a continuous phase of a firstthermoplastic polymer and a discontinuous or disperse phase of a secondthermoplastic polymer. Either or both of the first or second polymersmay be a birefringent material. In one embodiment, a diffuselyreflective polarizer includes more than one continuous phase and/or morethan one disperse phase.

In another embodiment, oriented optical films made by methods of thepresent disclosure include compensators and retarders. An exemplaryembodiment is an “a-plate,” which is a birefringent optical element,such as, for example, a plate or film, having its principle optical axiswithin the x-y plane of the optical element. Positively birefringenta-plates can be fabricated using, for example, uniaxially stretchedfilms of polymers such as, for example, polyvinyl alcohol, or uniaxiallyaligned films of nematic positive optical anisotropy liquid crystalpolymer (LCP) materials. Negatively birefringent a-plates can be formedusing uniaxially aligned films of negative optical anisotropy nematicLCP materials, including for example discotic compounds.

When the volume fraction for binary blends of high polymers of roughlyequivalent viscosity is greater than about 40% and approaches 50%, thedistinction between the disperse and continuous phases becomesdifficult, as each phase becomes continuous in space. Depending upon thematerials of choice, there may also be regions where the first phaseappears to be dispersed within the second, and vice versa. For adescription of a variety of co-continuous morphologies and for methodsof evaluating, analyzing, and characterizing them, see Sperling and thereferences cited therein (L. H. Sperling, “Microphase Structure,”Encyclopedia of Polymer Science and Engineering, 2nd Ed., Vol. 9,760-788, and L. H. Sperling, Chapter 1, “Interpenetrating PolymerNetworks: An Overview,” Interpenetrating Polymer Networks, edited by D.Klempner, L. H. Sperling, and L. A. Utracki, Advances in ChemistrySeries #239, 3-38, 1994).

One set of acceptable parabolic trajectories that is nearly orsubstantially uniaxial in character can be determined by the followingmethod. This described method determines the “right” boundary trajectorydirectly, and the “left” boundary trajectory is taken as a mirror image.First, a condition is set by defining an instantaneous functionalrelationship between TDDR measured between the opposing boundarytrajectories and MDDR defined as the cosine of the non-negativedivergence angle of those boundary trajectories, over a chosen range ofTDDR.

Next, the geometry of the problem is defined as described in thediscussion of the parabolic trajectories. x₁ is defined as the initialhalf distance between the boundary trajectories and a ratio (x/x₁) isidentified as the instantaneous TDDR, where x is the current x positionof a point on the boundary trajectory. Next, the instantaneousfunctional relationship between the TDDR and MDDR is converted to arelationship between TDDR and the divergence angle. When a specificvalue of U is chosen, the equations above provide a specificrelationship between MDDR and TDDR which can then be used in thealgorithm to specify the broader class of boundary trajectories thatalso includes the parabolic trajectories as a limiting case when Uapproaches unity. Next, the boundary trajectory is constrained tosatisfy the following differential equation:

d(x/x ₁)/d(y/x ₁)=tan(θ)

where tan(θ) is the tangent of the divergence angle θ, and y is the ycoordinate of the current position of the opposing point on the rightboundary trajectory corresponding to the given x coordinate. Next, thedifferential equation may be solved, e.g. by integrating 1/tan(θ) alongthe history of TDDR from unity to the maximum desired value to obtainthe complete coordinate set {(x,y)} of the right boundary trajectory,either analytically or numerically.

As another example of acceptable trajectories, a class of in-planetrajectories can be described in which the parabolic trajectory is usedwith smaller or larger initial effective web TD length. If x₁ is half ofthe separation distance between the two opposing boundary trajectoriesat the inlet to the primary stretching region 66 (i.e. the initial filmTD dimension minus the selvages held by the grippers which is theinitial half distance between opposing boundary trajectories), then thisclass of trajectories is described by the following equation:

±(x)/(x ₁)=(¼)(x ₁ /x ₀)(y/x ₁)²+1

where x₁/x₀ is defined as a scaled inlet separation. The quantity x₀corresponds to half of the separation distance between two opposingtracks required if the equation above described a parabolic track thatprovided a perfectly uniaxial stretch. The scaled inlet separation,x₁/x₀, is an indication of the deviation of the trajectory from theuniaxial condition. In one embodiment, the distance between the twoopposing tracks in the primary stretching zone is adjustable, asdescribed above, allowing for the manipulation of the trajectory toprovide values of U and F (described below) different than unity. Othermethods of forming these trajectories can also be used including, forexample, manipulating the shape of the trajectories using track shapecontrol units or by selecting a fixed shape that has the desiredtrajectory.

For super-uniaxial stretches, the severity of the wrinkling can bequantified using the concept of overfeed. The overfeed, F, can bedefined as the uniaxial MDDR (which equals (TDDR)^(−1/2)) divided by theactual MDDR. If the actual MDDR is less than the uniaxial MDDR, theoverfeed F is less than unity and the MDDR is under-relaxed resulting ina U less than unity. If F is greater than unity, the stretch issuper-uniaxial and the MDDR is over-relaxed relative to the uniaxialcase. At least a portion of the extra slack can be accommodated as awrinkle because the compressive buckling threshold is typically low forthin, compliant films. When F is greater than unity, the overfeedcorresponds at least approximately to the ratio of the actual filmcontour length in the wrinkles along MD to the in-plane contour lengthor space.

Because of the relationship between TDDR and MDDR in the case ofconstant density, F can be written as:

F=1/(MDDR×TDDR^(1/2))

Typically, F is taken as density independent for design purposes. Largevalues of F anytime during the process can cause large wrinkles that canfold over and stick to other parts of the film, thereby causing defects.In at least some embodiments, the overfeed, F, remains at 2 or lessduring the stretch to avoid or reduce severe wrinkling or fold-over. Insome embodiments, the overfeed is 1.5 or less throughout the course ofthe stretch. For some films, a maximum value of F of 1.2 or even 1.1 isallowed throughout the stretch.

For at least some embodiments, particularly embodiments with U>1 throughthe entire stretch, rearranging the definition of overfeed provides arelative bound on a minimum MDDR given a current TDDR:

MDDR>1/(F _(max)×TDDR^(1/2))

where F_(max) can be chosen at any preferred level greater than unity.For example, F can be selected to be 2, 1.5, 1.2, or 1.1, as describedabove.

When the over-feed is less than unity, there is effectively morein-plane space along MD than is desired for the truly uniaxial stretchand the MDDR may be under-relaxed and causing MD tension. The result canbe a U value less than unity. Using the relationships between U, F, MDDRand TDDR there is a corresponding correlation between U and F whichvaries with TDDR. At a critical draw ratio of 2, a minimum U valuecorresponds to a minimum overfeed F of about 0.9. For at least someboundary trajectories including boundary trajectories in which U>1 forthe entire stretch, MDDR can be selected to remain below a certain levelduring a final portion of stretch, e.g.:

MDDR<1/(F _(min)×TDDR^(1/2))

where F_(min) is 0.9 or more for a final portion of stretch after a drawratio of 2.

As an example, trajectories can be used in which MDDR<(TDDR)^(−1/2)(i.e., U>1) throughout the stretch, F_(max) is 2, and the film isstretched to a TDDR of at 4. If the trajectories are coplanar, then thefilm is stretched to a TDDR of at least 2.4 and often at least 5.3. IfF_(max) is 1.5, then the film is stretched to a TDDR of at least 6.8. Ifthe trajectories are coplanar, then the film is stretched to a TDDR ofat least 2.1 and often at least 4.7. If F_(max) is 1.2, then the film isstretched using coplanar trajectories to a TDDR of at least 1.8 andoften at least 4.0. For coplanar or non-coplanar boundary trajectories,if no limit is placed on F, then the film is stretched to a TDDR ofgreater than 4 and often at least 6.8. In another example, coplanartrajectories can be used in which (F_(min))(MDDR)<(TDDR)^(−1/2)throughout the stretch, F_(max) is 2, F_(min) is 0.9, and the film isstretched to a TDDR of at least 4.6 and often at least 6.8. If F_(max)is 1.5, then the film is stretched to a TDDR of at least 4.2 and oftenat least 6.1, If F_(max) is 1.2, then the film is stretched to a TDDR ofat least 3.7 and often at least 5.4. If no limit is placed on F, thenthe film is stretched to a TDDR of at least 8.4. A boundary trajectorycan also be used in which (F_(min))(MDDR)<(TDDR)^(−1/2) throughout thestretch, F_(max) is 1.5, F_(min) is 0.9, and the film is stretched to aTDDR of at least 6.8.

Other useful trajectories can be defined using F_(max). Usefultrajectories include coplanar trajectories where TDDR is at least 5, Uis at least 0.85 over a final portion of the stretch after achieving aTDDR of 2.5, and F_(max) is 2 during stretching. Useful trajectoriesalso include coplanar trajectories where TDDR is at least 6, U is atleast 0.7 over a final portion of the stretch after achieving a TDDR of2.5, and F_(max) is 2 during stretching.

Yet other useful coplanar trajectories include those in whichMDDR<TDDR^(−1/2)<(F_(max))(MDDR) during a final portion of the stretchin which TDDR is greater than a critical value TDDR′. The followingprovides minimum draw ratios that should be achieved for the trajectoryin some exemplary embodiments. When TDDR′ is 2 or less, then forF_(max)=2, the minimum draw is 3.5; for F_(max)=1.5, the minimum draw is3.2; and for F_(max)=2, the minimum draw is 2.7. When TDDR′ is 4 orless, then for F_(max)=2, the minimum draw is 5.8; for F_(max)=1.5, theminimum draw is 5.3; and for F_(max)=1.2, the minimum draw is 4.8. WhenTDDR′ is 5 or less, then for F_(max)=2, the minimum draw is 7; forF_(max)=1.5, the minimum draw is 6.4; and for F_(max)=1.2, the minimumdraw is 5.8.

In general, a variety of acceptable trajectories can be constructedusing curvilinear and linear tracks so that the overfeed remains below acritical maximum level throughout the stretching to prevent fold-overdefects while remaining above a critical minimum level to allow thedesired level of truly uniaxial character with its resulting properties.

A variety of sub-uniaxial and super-uniaxial trajectories may be formedusing the parabolic shape. FIG. 19 illustrates examples of differentlevels of minimum U after a critical TDDR and demonstrate differentmaximum overfeeds up to a final desired TDDR. The curves are representedby coordinates x and y as scaled by x₁, half the initial separationdistance of the tracks. The scaled x coordinate, the quantity NO, istherefore equal to the TDDR. Curve 120 is the ideal case with a value ofx₁/x₀ of 1.0. Curve 122 is the parabolic case with a value of x₁/x₀ of0.653 in which U remains greater than 0.70 above a draw ratio of 2.5.Curve 124 is the parabolic case with a value of x₁/x₀ of 0.822 in whichU remains above 0.85 after a draw ratio of 2.5.

Curves 126. 128, and 130 illustrate various levels of overfeed. Theoverfeed, TDDR and scaled inlet width are related by:

x ₁ /x ₀=(F ²(TDDR)−1)/(TDDR−1)

It follows directly that the overfeed increases with increasing TDDR inthe parabolic trajectories described here. Curve 126 is the paraboliccase with a value of x₁/x₀ of 1.52 in which the overfeed remains below1.2 up to a final draw ratio of 6.5. Curve 128 is the parabolic casewith a value of x₁/x₀ of 2.477 in which the overfeed remains below 1.5up to a final draw ratio of 6.5. Curve 130 is the parabolic case with avalue of x₁/x₀ of 4.545 in which the overfeed remains below 2 up to afinal draw ratio of 6.5. The level of overfeed is a function of thefinal draw ratio in these cases. For example, using a value of x₁/x₀ ofonly 4.333 rather than 4.545 allows stretching to a final TDDR of 10while keeping the overfeed under 2.

For the parabolic trajectories, a relationship allows the directcalculation of MDDR at any given TDDR for a fixed scaled inlet width:

MDDR=(TDDR(x ₁ /x ₀)+(1−x ₁ /x ₀))−^(1/2)

One observation is that the relationship between MDDR and TDDR is not anexplicit function of the y position. This allows the construction ofcomposite hybrid curves comprising sections of parabolic trajectoriesthat are vertically shifted in y/x₁. FIG. 20 illustrates one method. Aparabolic trajectory for the initial portion of the stretch is chosen,curve 132, and a parabolic trajectory is chosen for the final portion,curve 134. The initial curve 132 is chosen to provide a super-uniaxialstretch with a maximum overfeed of 2.0 at a draw ratio of 4.5. Curve 132has a scaled inlet width of 4.857. The final curve 134 is chosen to be asub-uniaxial stretch with a minimum U of 0.9 at the 4.5 draw ratio.Curve 134 has a scaled inlet width of 0.868. The actual track or railshape follows curve 132 up to TDDR of 4.5 and then continues on curve136 which is a vertically shifted version of curve 134. In other words,a trajectory can have an initial stretching zone with tracks having afunctional form corresponding to:

±(x)/(x ₁)=(¼)(x ₁ /x ₀)(y/x ₁)²+1

and then a later stretching zone with tracks having a functional formcorresponding to:

±(x)/(x ₂)=(¼)(x ₂ /x ₀)((y−A)/x ₂)²+1;

where x₁ and x₂ are different and A corresponds to the vertical shiftthat permits coupling of the trajectories. Any number of parabolicsegments may be combined in this manner.

The parabolic trajectories, and their composite hybrids, can be used toguide the construction of related trajectories. One embodiment involvesthe use of linear segments to create trajectories. These linearapproximations can be constructed within the confines of parabolictrajectories (or composite hybrids) of maximum overfeed and minimumoverfeed (or minimum U) at a chosen TDDR′ larger than a critical drawratio, TDDR*. Values for TDDR* can be selected which relate to the onsetof strain-induced crystallinity with examples of values of 1.5, 2, and2.5 or may be related to elastic strain yielding with lower values of1.2 or even 1.1. The range of TDDR* generally falls between 1.05 and 3.Portions of the rail or track below TDDR* may not have any particularconstraints on minimum overfeed or U and may fall outside the confinesof the constraining parabolic trajectories.

In FIG. 21, curve 138 is chosen to be the constraining parabolictrajectory of minimum overfeed at the chosen draw ratio, TDDR′,illustrated here at a value of 6.5. For illustration, the minimumoverfeed constraining parabolic trajectory has been chosen as the idealcurve with a scaled inlet width of unity. Using the relationship betweenoverfeed, TDDR and scaled inlet width, curve 140 is identified as theconstraining parabolic trajectory of maximum overfeed where the maximumvalue of F is 2.0 at the TDDR value of 6.5. Curve 140 is now verticallyshifted to form curve 142 so that the two constraining parabolictrajectories meet at the chosen TDDR′ of 6.5. It should be remarked thatcurves 140 and 142 are equivalent with respect to stretching character.Curve 142 merely delays the stretch until a later spatial value of y/x₁of 2.489. An approximation of linear or non-parabolic curvilinearsegments will tend to lie between these constraining trajectories aboveTDDR*.

Unlike parabolic trajectories that possess increasing divergence angleswith increasing TDDR, linear trajectories have a fixed divergence angle.Thus the overfeed decreases with increasing TDDR along a linear segment.A simple linear approximation can be constructed by choosing a line witha divergence angle equal to the desired minimum overfeed at the chosenTDDR. The line segment may be extrapolated backwards in TDDR until theoverfeed equals the maximum allowed. A subsequent linear segment isstarted in similar fashion. The procedure is repeated as often asnecessary or desired. As the maximum overfeed decreases, the number ofsegments needed for the approximation increases.

When the TDDR drops below TDDR*, any number of methods may be used tocomplete the track or rail as long as the constraint on maximum overfeedis maintained. In FIG. 21, curve 144 is a linear approximationconstrained by a maximum overfeed of 2. Because of this large maximumoverfeed, it comprises only two linear sections. The final linearsegment extends all the way backwards from the chosen TDDR of 6.5 to alower TDDR of 1.65. In this case, TDDR* is taken as 2. Without aconstraint on U below a TDDR of 2, one method of finishing the track isto extrapolate a second linear segment from TDDR at 1.65 back to TDDR ofunity at the y/x₁ zero point. Note that this causes the second segmentto cross the lower constraining parabola, since the constraint is noteffective below TDDR*.

In FIG. 21, curve 146 is the result of using a tighter value for themaximum overfeed of 1.5. Here the constraining parabolic trajectory ofmaximum overfeed is not shown. Three linear segments are required. Thefirst segment extends backwards from TDDR of 6.5 to TDDR of 2.9. Thesecond segment assumes a divergence angle equal to the constrainingparabolic trajectory of minimum overfeed at this TDDR value of 2.9 andextends backwards to a TDDR of 1.3. This second segment ends belowTDDR*. The final segment completes the track or rail shape for curve 146using a different method than that used for curve 144. Here the sameprocedure for the last segment is used as for the previous segments,resulting in a delay of the onset of stretching with a higher y/x₁value. A third method of completing the track is to set the overfeed tothe maximum at the initial TDDR of unity.

General, non-linear and non-parabolic trajectories fitting therequirements of the present disclosure can be constructed using theconstraining parabolic trajectories. The maximum overfeed constrainingparabolic trajectory is the curve of minimum slope, i.e. maximumdivergence angle, as a function of TDDR. The minimum overfeedconstraining parabolic trajectory is the curve of maximum slope, i.e.minimum divergence angle, as a function of TDDR. In general, curves canbe extrapolated backwards from the chosen TDDR′ using any function ofslope that lies between the constraining bounds.

A simple method for defining a function for the slope that lies betweenthese constraints is to take a simple linear combination of known curveswithin the envelope. Curve 148 in FIG. 21 illustrates this simplemethod. In this example, curve 148 is formed by a linear combination ofthe maximum overfeed constraining parabolic trajectory, curve 142, andthe linear approximation to it, curve 144, with the linear weights of0.7 and 0.3, respectively. In general, functions that are not simplelinear combinations can also be used.

The aforementioned method for describing the various non-parabolictrajectories of the present disclosure can be applied over differentsections of the track, e.g. the example of FIG. 21 for TDDR up to 6.5may be combined with another section for TDDR over 6.5 with differentrequirements and therefore different maximum and minimum constrainingtrajectories over that higher range of TDDR. In this case, the TDDR′ ofthe previous section of lower stretch takes on the role of TDDR*. Ingeneral, TDDR′ may be chosen across the range of desired stretching.Various sections may be used to account for the various phenomenon ofstretching, such as yielding, strain-induced crystallization, onset ofnecking or other stretch non-uniformity, onset of strain-hardening or toaccount for the development of various properties within the film.Typical break points include those for TDDR*, the range of 3 to 7 forstrain-hardening in polyesters, and typical final draw values in therange of 4 to 10 or more.

The procedures for determining boundary trajectories for the presentdisclosure and the method of extrapolating backwards to lower TDDR froma chosen TDDR′ may be used in an analogous method of extrapolatingforward to higher TDDR from a chosen TDDR″. Again, two constrainingtrajectories are formed, joined at the lowest chosen TDDR″. A convenientvalue for TDDR″ is the initial TDDR of unity. In this method, theconstraining trajectory of minimum overfeed or U lies above the maximumoverfeed curve. FIG. 20 exhibits an example of this method in which thehybrid curve 136 lies between the minimum overfeed constraint, curve134, and the maximum overfeed constraint, curve 132.

Still another class of boundary trajectories can be defined and may, insome embodiments, be useful in suppressing residual wrinkles. Becausethe uniaxial condition in the absence of shear provides a principal MDstress of zero, it is anticipated, using finite strain analysis, thatthe principal MD stress will actually go into slight compression underthese conditions. Using finite strain analysis and a Neo-Hookean elasticsolid constitutive equation, it is discovered that a suitable criterionfor preventing compressive stresses may optionally be given by thefollowing equation:

((TDDR)(MDDR))⁻⁴+((TDDR)(MDDR))²−(TDDR)⁻²−(MDDR)⁻²−sin²(θ)((TDDR)(MDDR))⁻²=0

where MDDR is the cosine of the divergence angle. This optional methodof the present disclosure then specifies this class of boundarytrajectories.

As indicated above, the film may be stretched out-of-plane usingout-of-plane boundary trajectories, i.e. boundary trajectories that donot lie in a single Euclidean plane. There are innumerable, butnevertheless particular, boundary trajectories meeting relationalrequirements of this preferred embodiment of the present disclosure, sothat a substantially uniaxial stretch history may be maintained usingout-of-plane boundary trajectories. The boundaries may be symmetrical,forming mirror images through a central plane, e.g. a plane comprisingthe initial center point between the boundary trajectories, the initialdirection of film travel and the initial normal to the unstretched filmsurface. In this embodiment, the film may be stretched between theboundary trajectories along a cylindrical space manifold formed by theset of line segments of shortest distance between the two opposingboundary trajectories as one travels along these boundary trajectoriesat equal rates of speed from similar initial positions, i.e., colinearwith each other and the initial center point.

The trace of this ideal manifold on the central plane thus traces outthe path of the film center for an ideal stretch. The ratio of thedistance along this manifold from the boundary trajectory to thiscentral trace on the central plane to the original distance from thestart of the boundary trajectory to the initial center point is theinstantaneous nominal TDDR across the film spanning the boundarytrajectories, i.e. the ratios of the half-distances between the currentopposing points on the boundary trajectories and the half-distancesbetween the initial positions of the opposing points on the boundarytrajectories. As two opposing points move at constant and identicalspeeds along the opposing boundary trajectories, the correspondingcenter point on the central trace changes speed as measured along thearc of the central trace, i.e. the curvilinear MD. In particular, thecentral trace changes in proportion with the projection of the unittangent of the boundary trajectory on the unit tangent of the centraltrace.

The classes of trajectories described above are illustrative and shouldnot be construed as limiting. A host of trajectory classes areconsidered to lie within the scope of the present disclosure.

As indicated above, the primary stretching region can contain two ormore different zones with different stretching conditions. For example,one trajectory from a first class of trajectories can be selected for aninitial stretching zone and another trajectory from the same first classof trajectories or from a different class of trajectories can beselected for each of the subsequent stretching zones.

Although exemplary embodiments of the present disclosure encompass allboundary trajectories comprising a minimum value of U>0, typicalembodiments of the present disclosure include all nearly orsubstantially uniaxial boundary trajectories comprising a minimum valueof U of about 0.2, about 0.5, about 0.7, more preferably approximately0.75, still more preferably about 0.8 and even more preferably about0.85. The minimum U constraint may be applied over a final portion ofthe stretch defined by a critical TDDR preferably of about 2.5, stillmore preferably about 2.0 and more preferably about 1.5. In someembodiments, the critical TDDR can be about 4 or 5. Above a criticalTDDR, certain materials, e.g. certain monolithic and multilayer filmscomprising orientable and birefringent polyesters, may begin to losetheir elasticity or capability of snap back because of the developmentof structure such as strain-induced crystallinity. The critical TDDR maycoincide with a variety of material and process (e.g. temperature andstrain rate) specific events such as the critical TDDR for the onset ofstrain-induced crystallization. The minimum value of U above such acritical TDDR can relate to an amount of non-uniaxial character set intothe final film.

A variety of boundary trajectories are available when U is subuniaxialat the end of the stretching period. In particular, useful boundarytrajectories include coplanar trajectories where TDDR is at least 5, Uis at least 0.7 over a final portion of the stretch after achieving aTDDR of 2.5, and U is less than 1 at the end of the stretch. Otheruseful trajectories include coplanar and non-coplanar trajectories whereTDDR is at least 7, U is at least 0.7 over a final portion of thestretch after achieving a TDDR of 2.5, and U is less than 1 at the endof the stretch. Useful trajectories also include coplanar andnon-coplanar trajectories where TDDR is at least 6.5, U is at least 0.8over a final portion of the stretch after achieving a TDDR of 2.5, and Uis less than 1 at the end of the stretch. Useful trajectories includecoplanar and non-coplanar trajectories where TDDR is at least 6, U is atleast 0.9 over a final portion of the stretch after achieving a TDDR of2.5, and U is less than 1 at the end of the stretch.

Useful trajectories also include coplanar and non-coplanar trajectorieswhere TDDR is at least 7 and U is at least 0.85 over a final portion ofthe stretch after achieving a TDDR of 2.5.

In some embodiments, a small level of MD tension is introduced into thestretching process to suppress wrinkling. Generally, although notnecessarily, the amount of such MD tension increases with decreasing U.In some embodiments, it is useful to increase the tension as the stretchproceeds. For example, a smaller value of U earlier in the stretch maytend to set more non-uniaxial character into the final film. Thus it maybe advantageous to combine the attributes of various trajectory classesinto composite trajectories. For example, a uniaxial parabolictrajectory may be preferred in the earlier portions of the stretch,while the later portions of the stretch may converge on a differenttrajectory. In another arrangement, U may be taken as a non-increasingfunction with TDDR. In still another arrangement, the overfeed, F, maybe a non-increasing function with TDDR after a critical draw ratio of,for example, 1.5, 2, or 2.5.

The uniaxial parabolic trajectory assumes a uniform spatial stretchingof the film. Good spatial uniformity of the film can be achieved withmany polymer systems with careful control of the crossweb and downwebcaliper (thickness) distribution of the initial, unstretched film orweb, coupled with the careful control of the temperature distribution atthe start of and during the stretch. For example, a uniform temperaturedistribution across the film initially and during stretch on a film ofinitially uniform caliper should suffice in most cases. Many polymersystems are particularly sensitive to non-uniformities and will stretchin a non-uniform fashion if caliper and temperature uniformity areinadequate.

Non-uniform film stretching can occur for a variety of reasonsincluding, for example, non-uniform film thickness or other properties,non-uniform heating, etc. In many of these instances, portions of thefilm near the gripping members stretch faster than those near thecenter. This creates an MD tension in the film that can limit ability toachieve a final uniform MDDR. One compensation for this problem is tomodify the parabolic or other uniaxial trajectory to present a lowerMDDR. In other words, MDDR<(TDDR)^(−1/2) for all or a portion of thestretch.

In one embodiment, a modified parabolic or other uniaxial trajectory isselected in which MDDR<(TDDR)^(−1/2), corresponding to a largerdivergence angle, for all of the stretch. In at least some instances,this condition can be relaxed because a U value of less than unity isacceptable for the application. In such instances, a modified parabolicor other uniaxial trajectory is selected in which(0.9)MDDR<(TDDR)^(−1/2).

In another embodiment, a modified parabolic or other uniaxial trajectoryis selected in which MDDR<(TDDR)^(−1/2) for an initial stretching zonein which the TDDR is increased by at least 0.5 or 1. A differenttrajectory is then maintained for the remainder of the stretch. Forexample, a later stretching zone (within the stretching region 34) wouldhave a parabolic or other uniaxial trajectory in which MDDR is equal toor approximately equal to (within ±5% and, preferably, within ±3%)(TDDR)^(−1/2). As an example, the initial stretching zone can accomplisha TDDR level up to a desired value. In one embodiment, this desiredvalue is typically no more than about 4 or 5. The later stretching zonecan then increase the TDDR from the desired value of the initialstretching zone (or from a higher value if there are interveningstretching zones). Generally, the later stretching zone is selected toincrease the TDDR value by 0.5 or 1 or more.

Again, in at least some instances, the MDDR and TDDR relationship can berelaxed because a U value of less than unity is acceptable for theapplication. In such instances, the modified parabolic or other uniaxialtrajectory of the initial stretching zone is selected in which(0.9)MDDR<(TDDR)^(−1/2).

The heat set procedure of the present disclosure may be performed duringvarious portions of the stretching process. In one embodiment, the film32 may be heat set following stretching and hand-off to a takeawaysystem, i.e. in a heated takeaway zone. In another embodiment, the film32 may be heat set in an on-line zone subsequent to initial quenchingand setting of the film 32, e.g. in a separate oven device that re-heatsthe film 32. In yet another embodiment, the film 32 may be heat setafter winding into a roll after the initial process, e.g. in a separateoven device not connected on-line to the stretching apparatus 50.

During heat setting, the draw ratio used for stretching the film may beincreased, maintained, or decreased compared to the draw ratio used toinduce the substantially uniaxial orientation. In other words, the film32 may be further stretched or the stretching may be relaxed, e.g. witha toe-in (reduction in stretch ratio) as provided by an edge grippingmechanism in any of these steps. For example, the take-away can betoed-in or the film 32 may be gripped in a clip system and conveyed byit through the separate oven device with a variable width profile, e.g.a toe-in or an increase in stretch perhaps also followed by a subsequenttoe-in. The heat set procedure may be performed with the filmcontinuously gripped and under tension, e.g. using an edge grippingprofile of increase or reduced tension or both as controlled by theseparation profile of the opposing grips, or gripped continuously ordiscontinuously along a system of converging and diverging rails. Thefilm can also be unconstrained at the edges.

Heat setting can also be combined with other film post-processing. Forexample, the film may be coated and dried or cured in an oven with someheat setting effect.

In some embodiments, such as that illustrated in FIG. 22, a takeawaysystem 150 can use any film conveyance structures such as tracks 150,152 with gripping members such as, for example, opposing sets of beltsor tenter clips. TD shrinkage control can be accomplished using tracks152, 154 which are angled (as compared to parallel tracks 156, 158 thatcould be used in other embodiments of a suitable take-away system 150).For example, the tracks 152, 154 of the take-away system 150 can bepositioned to follow a slowly converging path (in one embodiment, makingan angle θ of no more than about 5°) through at least a portion of thepost conditioning region 70 to allow for TD shrinkage of the film 32with cooling. The tracks 152, 154 in this configuration allow thecontrol of TD shrinkage to increase uniformity in the shrinkage. Inother embodiments, the two opposing tracks 152, 154 can be divergingtypically at an angle of no more than about 3° although wider angles canbe used in some embodiments. This can be useful to increase the MDtension of the film 32 in the primary stretching region 66 to, forexample, reduce property non-uniformity such as the variation ofprincipal axes of refractive index across the film 32.

In some embodiments, the position of the take-away system 150 can beadjustable to vary the position along the stretching apparatus 50 atwhich the take-away system 150 grips the film 32, as illustrated in FIG.23. This adjustability provides one way to control the amount ofstretching to which the film 32 is subjected. Film 32 received by tracks156′, 158′ of a take-away system earlier in the stretch (shown by dottedlines in FIG. 23) will generally have a smaller TDDR than would filmreceived by a tracks 156, 158 of a take-away system 150 positioned laterin the stretch (shown in solid lines in FIG. 23). The take-away system150 can also, optionally, allow adjustment in the distance between theopposing tracks 152, 154, 156, 158 of the take-away system 150. Inaddition, the take-away system 150 can also, optionally, be configuredto allow adjustment in the length of the take-away system 150.

Another example of a possible take-away system 150, illustrated in FIG.25, includes at least two different regions with separated tracks 152,154, 156, 158. These regions can be formed using two separate sets 152,154 and 156, 158 of opposing tracks as illustrated in FIG. 24. In oneembodiment, illustrated in FIG. 24, the first region can include tracks152, 154 that are disposed at a convergence angle to provide TDshrinkage control and the tracks 156, 158 in the second regions can beparallel. In other embodiments, the opposing tracks of the two differentregions can be set at two different convergence angles to provide TDshrinkage control, as described above, or the first region can haveparallel tracks and the second region have tracks disposed at aconvergence angle to provide TD shrinkage control. Alternatively oradditionally, the two different tracks can be set at two differenttakeaway speeds to decouple the primary stretching region 66 from atakeaway region that applies tension to remove wrinkles.

In one embodiment of the take-away system 150 illustrated in FIG. 24,the tracks 156′, 158′ are nested within the opposing tracks 152, 154prior to receiving the film 32. When the film 32 is initially receivedby the opposing tracks 152, 154, the tracks 156′, 158′ move to theposition 156, 158 illustrated in FIG. 24. In other embodiments, theopposing tracks 152, 154, 156, 158 are positioned as illustrated in FIG.24 (i.e., not nested) in the absence of any film 32. Another example ofa take-away system is illustrated in FIG. 25. In this example, thetracks 152, 154 of the take-away system are angled with respect to thecenterline of the film 32 as the film 32 is conveyed through the tracks54 of the primary stretching region 66.

The angle of the two opposing conveyance mechanisms 152, 154 can be thesame, for example, an angle β, or the angle can be different and can bedescribed as β+ε for one track 152 and β−ε for the other track 154.Typically, β is at least about 1° and can be an angle of about 5°, 10°,or 20° degrees or more. The angle ε corresponds to the converging ordiverging angle described above to provide TD shrinkage control, forexample. In some embodiments, the tracks 54 in the primary stretchingzone 66 can also be disposed at an angle ϕ and the tracks 152, 154 areangled at ϕ+β+ε and ϕ+β−ε as illustrated in FIG. 25. An angled take-awaysystem 150, primary stretching zone 66, or both can be useful to providefilms 32 where the principal axis or axes of a property of the film 32,such as the refractive index axes or tear axis, is angled with respectto the film 32. In some embodiments, the angle that the take-away system150 makes with respect to the primary stretching zone 66 is adjustablemanually or mechanically using a computer-controlled driver or othercontrol mechanism or both.

In some embodiments using an angled take-away system 150, the twoopposing tracks 152, 154 are positioned to receive film 32 having thesame or substantially similar TDDR (where the dotted line 160 indicatesfilm 32 at the same TDDR), as illustrated in FIG. 25. In otherembodiments, the two opposing tracks 152, 154 are positioned to receivethe film 32 so that the TDDR is different for the two opposing tracks152, 154 (the dotted line 160 of FIG. 26 indicates film 32 at the sameTDDR), as illustrated in FIG. 26. This latter configuration can providea film 32 with properties that change over the TD dimension of the film32.

Referring back to FIG. 10, release of the selvages from a continuousgripping mechanism can be done continuously; however, release fromdiscrete gripping mechanisms, such as tenter clips, should preferably bedone so that all the material under any given clip is released at once.Discrete release mechanisms may cause larger upsets in stress that maybe felt by the stretching web upstream. In order to assist the action ofthe isolating takeaway device, it is preferred in one embodiment to usea continuous selvage separation mechanism in the device, such as, forexample, the “hot” slitting of the selvage 76 from the central portionof a heated, stretched film.

In one embodiment, the slitting location 78 is preferably located nearenough to the “gripline,” e.g. the isolating takeaway point of firsteffective contact by the gripping members of the take-away system, tominimize or reduce stress upsets upstream of that point. If the film isslit before the film is gripped by the take-away system, instabletakeaway can result, for example, by film “snapback” along TD. The filmis thus preferably slit at or downstream of the gripline. Slitting is afracture process and, as such, typically has a small but naturalvariation in spatial location. Thus it may be preferred to slit slightlydownstream of the gripline to prevent any temporal variations inslitting from occurring upstream of the gripline. If the film is slitsubstantially downstream from the gripline, the film between thetakeaway and boundary trajectory will continue to stretch along TD.Since only this portion of the film is now stretching, it now stretchesat an amplified draw ratio relative to the boundary trajectory, creatingfurther stress upsets that could propagate upstream, for example,undesirable levels of machine direction tension propagating upstream.

The slitting is preferably mobile and re-positionable so that it canvary with the changes in takeaway positions needed to accommodatevariable final transverse draw direction ratio or adjustment of theposition of the take-away system. An advantage of this type of slittingsystem is that the draw ratio can be adjusted while maintaining thestretch profile simply by moving the take-away slitting point 78.

A variety of slitting techniques can be used including a heat razor, ahot wire, a laser, a focused beam of intense infrared (IR) radiation ora focused jet of heated air. In the case of the heated jet of air, theair may be sufficiently hotter in the jet to blow a hole in the film,such as by heat softening, melting, or controlled fracture under thejet. Alternatively, the heated jet may merely soften a focused sectionof the film sufficiently to localize further stretching imposed by thestill diverging boundary trajectories, thus causing eventual fracturedownstream along this heated line through the action of continued filmextension. The focused jet approach may be preferred in some cases,especially when the exhaust air can be actively removed, e.g. by avacuum exhaust, in a controlled fashion to prevent stray temperaturecurrents from upsetting the uniformity of the stretching process. Forexample, a concentric exhaust ring around the jet nozzle can be used.Alternatively, an exhaust underneath the jet, e.g. on the other side ofthe film, can be used. The exhaust may be further offset or supplementeddownstream to further reduce stray flows upstream into the stretchingzone.

Another attribute of one embodiment of the take-away system is a methodof speed and or MD tension control so that the film can be removed in amanner compatible with the output speed. In one embodiment, thistake-away system is used to pull out any residual wrinkles in the film.In one example, the wrinkles are initially pulled out during start up bya temporary increase in the takeaway speed above the output speed of thefinal, released portion of the stretched film. In another example, thewrinkles are pulled out by a constant speed above the output film MDspeed during continuous operation, such as in the case of asuper-uniaxial stretch in the final portion of stretch. In yet anotherexample, the speed of the takeaway is set above the MD velocity of thefilm along the boundary trajectories at the gripline. This can also beused to alter the properties of the film. This over-speed of thetakeaway can also reduce the final value of U; in some cases, this is aconsideration in the context of the final end use of the film.

The principles of MD and TD shrinkage control described above can alsobe applied to other stretching apparatuses including the conventionaltenter configuration illustrated in FIG. 2. FIG. 27 illustrates anembodiment in which the tracks 54 from a primary stretching region 66(such as the linear diverging tracks illustrated in FIG. 2) continueinto or through a portion of a post-conditioning region 70 (see FIG.10). The film is then optionally captured by an isolated takeaway system156, 158, if desired. The continuation of the tracks 54 can be used tocool the film and allow for shrinkage of the film.

In some embodiments, the continued tracks 162 follow a slowly convergingpath (making an angle θ of no more than about 5° in one embodiment)through at least a portion of the post conditioning region 70 to allowfor TD shrinkage of the film with cooling. The tracks in thisconfiguration allow the control of TD shrinkage to increase uniformityin the shrinkage. In some embodiments, the tracks 164 follow a moreaggressively converging path (making an angle ϕ of at least 15° in someembodiments, and typically in the range of 20° and 30°) through at leasta portion of the post conditioning region 70 to provide MD shrinkagecontrol of the film with cooling. In some embodiments as illustrated inFIG. 27, the post conditioning region 70 includes both slowly convergingtracks 162 and more aggressively converging tracks 164. In otherembodiments, only one set of tracks 162 or tracks 164 is used.

One useful measure of the uniaxial character of the film made inaccordance with a substantially uniaxial stretch process is the “extentof unixial character” described e.g. in U.S. Pat. No. 6,939,499,incorporated herein by reference. The approximate uniaxial character ofthe resulting film can be discerned by this process measurement. In onemeasure, the extent of uniaxial character is derived from the nominaldraw ratios as set by the bounding trajectories at the gripping edges ofthe device during stretching, as further modified by the conditions ofthe take away system. In another measure of the extent of uniaxialcharacter, the actual draw ratios of the film can be directly measured,e.g., by physical marking of the initial input cast web or film with agrid pattern of known size, and re-measurement after final filmformation, e.g. the factor ρ_(f).

The heat setting of the present disclosure allows for a greater range ofcontrol on the allowable set of refractive indices. In particular,higher values can be obtained at a fixed level of optical power asmeasured by the difference between nx and nu (discussed below), or astill higher nu value at lower levels of optical power can be obtained.

The heat treatment allows an additional measure of control on the set ofprincipal refractive indices initially resulting from the stretch andmay impart additional advantages such as, for example, improveddimensional stability including shrinkage control, enhanced creepresistance, improved imprint resistance, as well as enhanced tearresistance and other physical properties.

In some films comprising certain material systems, the heat treatmentmaintains or even improves the extent of uniaxial character in theresulting final film. In the case of optical films, this can maintain oreven improve performance in applications using non-normal incidentlight. For example, so-called off-angle color performance can bemaintained or improved in multilayer optical films (MOF) used forbrightness enhancement. MOF films used for polarizing beam splitting canalso be enhanced. The method can also be used to enhance orientation andperformance of microstructures formed on the surface of the film, e.g.in a polarizing beam splitting application.

For multilayer reflective polarizing films with a high degree ofuniaxial orientation (e.g. as achieved by a truly uniaxial stretchingprocess), higher levels of contrast can be achieved with a fixedmaterial constructions, i.e. fixed low index material. This can beapplied generally in applications that use these films, for example, inpolarizing beam splitter applications, e.g. as described in U.S. Pat.No. 6,609,795 and U.S. Patent Application Publication No. 2004/0227994using the films separately or together in stacks of two of more suchfilms.

Using a heat setting step, imprint resistance can be achieved using ahigh index skin layer. In many systems, the imprint resistance isincreased through the increased crystallinity in an oriented skin layer.The oriented skin layer may comprise a material similar to abirefringent layer in the optical stack of an MOF or it may include adifferent material suitably chosen to co-extrude and orient in the filmformation process.

Heat setting may also relieve the existence of “residual stresses” oftenremaining in the film after stretch, depending for example on therestraint conditions during or after heat setting. Reduced restraint,achieved by toe-in, for example, can contribute to stress reduction.This can lead to improved dimensional stability including lower levelsof shrinkage, lower levels of thermal expansion, and improved resistanceto warpage.

Other possible mechanical improvements upon heat setting may beincreased tear resistance or even increased inter-layer delaminationresistance. In some systems, high temperature heat setting near themelting point improves the interlayer adhesion. For example, it mayimprove the interfacial penetration between layers that can be perturbedduring the stretching process.

Further, when the method of the present disclosure is applied to a filmconstruction including a strain-induced crystallized polyester skinlayer, the imprint resistance of the film is improved. Light levels ofheat treatment do not significantly change the result; however, aheavier level of heat treatment creates a film with essentially nodenting.

In one embodiment, one or more of the heat set film layers remainsamorphous, leading to improved web handling and mechanical properties.In an exemplary embodiment, the amorphous layers comprise polycarbonateor a blend of polycarbonate and copolyester.

In certain polyester systems, heat setting provides higher optical poweror birefringence at a much lower draw ratio than is otherwise typicallyachieved by stretching alone. For example, polyesters such as PET, PENand compositions including both PET and PEN are typically stretched todraw ratios of 4, 5, 6 or higher. These materials may be stretched tojust above the strain-induced crystallization point and then heat set toachieve index values comparable to those higher draw ratios. As afurther example, a film with a microstructured surface may be stretchedin the cross direction, e.g. perpendicular to an elongate direction, ata substantially reduced draw ratio that may not overly destroy the shapeof the intended final structures. See, for example, copending, commonlyassigned U.S. Provisional Application Ser. No. 60/638,732; U.S.application Ser. No. 11/184,027; filed Dec. 23, 2004, incorporatedherein by reference. High levels of index can be achieved throughout themicrotextured structure along the cross direction as long as the onsetpoint of significant strain-induced crystallization has been surpassedthroughout the structure. This is especially useful for makingstructures with “fiber symmetric” index sets with high birefringencewhen the structures have a height varying or “profile” directionperpendicular to the stretch direction.

In the case of true or nearly uniaxially oriented films, the ny and nzare nearly identical, e.g. within a few hundredths of an index unit. Aninteresting and informative view of the space of allowable index setscan be obtained with the data reduction illustrated in FIG. 28.

To obtain the data plotted in FIG. 28, one first calculates the averageof the ny and nz indices actually obtained, regardless of processconditions. The average value, defined here as nu for “uniaxial index ofrefraction,” is a measure of the expected ny/nz value in a “virtual”truly uniaxial condition. In a multilayer optical film (MOF) polarizer,which includes alternating layers of birefringent and isotropicpolymeric materials, nu is the target pass state refractive index valueof the birefringent material to match to the isotropic index of thesecond, in some cases low index, material layer.

Second, one takes the difference between nx and nu. This difference isthe block state index difference, a measure of the reflective power oroptical power of the MOF polarizer in the virtual state.

Third, one plots the block state index difference versus the virtualtruly uniaxial pass state index nu.

FIG. 28 shows the resulting plot for a variety of stretch conditions,both truly uniaxial as described above and simply one-directional asperformed in a conventional tenter apparatus (FIGS. 2-3). The data covera wide range of effective molecular orientations as induced by stretchtemperature, rate and draw ratio, for polyesters spanning the range ofhomopolymer PEN, through the various “coPENs” to the homopolymer PET.The coPENS are expressed in terms of the ratio of mole percent PEN-likemoiety to mole percent PET-like moiety; for example, 85/15 co-polymer, aso-called “85/15 coPEN,” is a copolymer having 85 mole percent PEN-likemoiety and 15 mole percent PET-like moiety.

As a guide to the data, un-fitted, equally spaced, parallel lines arearranged in 10 weight % intervals across the composition range. So, thetop line follows the trend for 100% PEN. The next line represents 90%PEN and 10% PET; the following line represents 80% PEN and 20% PET, andso forth. The bottom line follows the trend for 100% PET.

The data remarkably falls close to these lines across the compositionrange. The effect of heat setting following a substantially uniaxialorientation is shown for the examples of PET and PEN and intermediatecompositions including both PET and PEN. With reference to PET, forexample, it can be seen that heat setting effectively moves the indexset up a line. Thus, after 100% PET is heat set, it optically behavesmore like a coPEN of 10% PEN and 90% PET. With reference to PEN, forexample, it can be seen that heat setting also effectively moves theindex set up. Thus, heat setting generally results in higher opticalpower (on the y-axis) for a given material, particularly at a givenmatching index (on the x-axis).

Moreover, for a given level of optical power (on the y-axis), heatsetting increases the matching index (on the x-axis) by about 0.01 ormore, compared to the untreated material. The greater control in nu fora given high index, birefirengent material, such as PET, for example,allows additional flexibility in the choice of materials, especially thesecond, in some cases low index, material in an optical film such as aMOF. Typically this second material is chosen to match the ny index ofthe oriented polyester in a polarizing film. Often this second materialis a copolyester chosen not only for its index matching but also for itsflow compatibility and mechanical attributes. Generally, a higher indextarget allows for a higher glass transition of such materials. Thus anadditional advantage is the additional dimensional stability obtained inMOF construction using a low index material with higher glass transitiontemperature. Moreover, the use of higher index materials allows for MOFconstruction with thinner and/or fewer layers.

Remarkably, unlike the asymmetric case, it appears that the heat settingof the present disclosure of substantially uniaxially stretched filmseither maintains or actually increases the extent of uniaxial characterof the films.

Another measure of uniaxial character is the relative birefringence,which compares the differences between the two similar refractiveindices, e.g. ny and nz, and between the significantly differentrefractive index, e.g. nx along the main stretch direction, and theaverage of the two similar indicies, e.g. nu. More precisely, therelative birefringence is given by:

Relative birefringence=|ny−nz|/|nx−nu|

where again nu is the average of the two similar indices of refraction,ny and nz, and the absolute values of the differences are taken. Therelative birefringence decreases as the uniaxial character of the filmsincreases.

In some exemplary embodiments, it appears that the heat setting of thepresent invention either maintains or actually decreases the relativebirefringence, particularly in certain polyester systems in which therelative birefringence before heat-setting is 0.1 or less. In otherexemplary embodiments, small increases in the relative birefringenceresult. In many embodiments, the final relative birefringence can be 0.1or less, even as an (absolute) in-plane birefringence (e.g. at 632.8 nm)of 0.1 or more is achieved. In other embodiments, the final relativebirefringence is 0.25, 0.2 or less.

The nature of the tension in the stretch direction (TD in one example)during the heat setting of the present example is an important factor inthe control of the index set. In general, a higher level of TD tensionthrough the heat setting process tends to increase nx more than ny/nz,while a lower or zero level of TD tension tends to increase the ny/nzwhile the nx increases slightly or even decreases in value. Thus, lowtension is useful in increasing the ny/nz value, while high tension isuseful in increasing optical power at a fixed nu level. Thus theprocesses described herein provide a method for contrast improvementwith a fixed material construction.

EXAMPLES General Notes on Examples

Two polyester-based constructions using two methods for making nearlytruly uniaxial film are exemplified. The first set of examples includemultilayered optical films (MOF) with a PET outer skin layer madethrough a batch tentering process as described herein with reference toFIG. 7. The second set of examples comprise MOF with a PEN outer skinmade through a parabolic tentering process such as that described inU.S. Pat. Nos. 6,939,499; 6, 916,440; 6,949,212; and 6,936,209.

Heat setting was performed in a batch stretching device in which thefilm could be constrained in the x and or y directions with edgegrippers. The stress in these constrained directions was also measuredduring the course of the heat setting. The films were heat set at 175°C. for three minutes, unless otherwise noted.

In the examples, the x direction is associated with the so-calledtransverse direction (TD) and the y direction is associated with themachine direction (MD).

Indices of refraction were measured using a Metricon Prism Coupler,available from Metricon, located in Piscataway, N.J. In general, twomodes can be measured with the device. The TE mode is used to measure anin-plane index of refraction. The TM mode is used to measure thethrough-thickness (for example, “z”) index of refraction. One maytherefore measure in the TM mode for various orientations of thein-plane states. For example, one may use the TM mode when the film isoriented to measure the in-plane index in the TD direction (noted asTD/z). As another example, the TM mode may be used with the film rotatedto measure the MD in-plane index (noted as MD/z). In general, thethrough-thickness indices should be about the same regardless of thein-plane orientation. However, discrepancies may arise due to thesharpness of the signal as a function of film orientation.

PET Examples:

MOF films with PET skin layers were highly extended using the batchtentering process described in FIG. 7 (Examples 1-7). The indexdevelopment in the PET skins after the stretching step was measuredusing an average of “top” and “bottom” sides using a Metricon Prismcoupler. Because of the extreme thinness of the outer PET layer, wavecoupling modes rather than a sharp knee were observed in the reflectedintensity vs. incidence angle plot. To improve precision, the index wasuniformly measured as the location of the leading edge of the firstobserved mode. This reasonably agrees with wave mode fitting undercertain circumstances, but may lead to a small understatement of nx inother circumstances. The ny and nz modes typically have less sharpreadings. Again the leading edge of the intensity drop was used.

Using this method the initial indices averaged 1.699, 1.541 and 1.539 at632.8 nm, for the nx, ny and nz respectively. The initial relativebirefringence using these index values is thus 0.013.

The overall results are presented in Table 1 below.

TABLE 1 Refractive In-plane Indices at orientation/TM 632.8 nm modemeasured TD TD TD MD MD MD TD/z PET Examples top bot ave top bot ave topExample 1: Slight Draw TD w heat Start, Before Heat Set 1.6939 1.70351.6987 1.5407 1.5402 1.5405 1.5391 5% stretch& Heat Set 1.7042 1.71061.7074 1.542 1.545 1.5435 1.5382 Diff 1st-Start 0.0087 0.0031 Example 2:Slight Draw TD w heat Start, Before Heat Set 1.6936 1.7027 1.6982 1.54031.5403 1.5403 1.5391 5% stretch& Heat Set 1.7012 1.7063 1.7038 1.54031.5403 1.5403 1.5374 Diff 1st-Start 0.0056 0.0000 Example 3: ConstrainTD/MD Start, Before Heat Set 1.6962 1.7043 1.7003 1.5409 1.5416 1.54131.5391 After 1st Heat Set 1.6995 1.7095 1.7045 1.5429 1.5421 1.54251.5418 Diff 1st-Start 0.0042 0.0012 Example 4: Taut start MD free Start,Before Heat Set 1.6918 1.7029 1.6974 1.5412 1.5414 1.5413 1.5394 After1st Heat Set 1.7037 1.7124 1.7081 1.5438 1.5423 1.5431 1.5414 Diff1st-Start 0.0107 0.0017 Example 5: Slight Slack at start TD Tension dev.MD free Start, Before Heat Set 1.6933 1.7027 1.6980 1.5412 1.5403 1.54081.5389 After 1st Heat Set 1.6999 1.7080 1.7040 1.5441 1.5436 1.54391.544 Diff End-Start 0.0059 0.0031 Example 6: TD no tension MD freeStart, Before Heat Set 1.6925 1.703 1.6978 1.5418 1.5418 1.5418 1.54After 1st Heat Set 1.6921 1.7003 1.6962 1.5476 1.5454 1.5465 1.5421 Diff1st-Start −0.0016 0.0047 After 2nd Heat Set 1.6928 1.703 1.6979 1.54721.5494 1.5483 1.5456 Diff 2nd-start 0.0002 0.0065 Example 7: No TensionMD free Start, Before Heat Set 1.6957 1.7033 1.6995 1.5412 1.5398 1.54051.5392 After 1st Heat Set 1.6952 1.7029 1.6991 1.5449 1.5449 1.54491.5427 Diff 1st-Start −0.0004 0.0044 Uniax limit MD/z TD/z MD/z ND MD −TD − PET Examples top bot bot ave Z MD Example 1: Slight Draw TD w heatStart, Before Heat Set 1.5392 1.5387 1.5385 1.5389 0.0016 0.1583 5%stretch& Heat Set 1.5385 1.5385 1.5402 1.5389 0.0046 0.1639 Diff1st-Start 0.0000 0.0031 0.0056 Example 2: Slight Draw TD w heat Start,Before Heat Set 1.5391 1.5378 1.5387 1.5387 0.0016 0.1579 5% stretch&Heat Set 1.5378 1.5371 1.5383 1.5377 0.0026 0.1635 Diff 1st-Start−0.0010 0.0010 0.0056 Example 3: Constrain TD/MD Start, Before Heat Set1.5385 1.5363 1.5365 1.5376 0.0036 0.1590 After 1st Heat Set 1.54181.5421 1.5405 1.5416 0.0010 0.1620 Diff 1st-Start 0.0039 −0.0027 0.0030Example 4: Taut start MD free Start, Before Heat Set 1.5396 1.5385 1.541.5394 0.0019 0.1561 After 1st Heat Set 1.5421 1.5416 1.5432 1.54210.0010 0.1650 Diff 1st-Start 0.0027 −0.0010 0.0090 Example 5: SlightSlack at start TD Tension dev. MD free Start, Before Heat Set 1.54091.5382 1.5403 1.5396 0.0012 0.1573 After 1st Heat Set 1.5434 1.54181.5423 1.5429 0.0010 0.1601 Diff End-Start 0.0033 −0.0002 0.0029 Example6: TD no tension MD free Start, Before Heat Set 1.5392 1.5391 1.53911.5394 0.0025 0.1560 After 1st Heat Set 1.5443 1.5446 1.5454 1.54410.0024 0.1497 Diff 1st-Start 0.0048 −0.0001 −0.0063 After 2nd Heat Set1.5458 1.5508 1.5501 1.5481 0.0002 0.1496 Diff 2nd-start 0.0087 −0.0022−0.0063 Example 7: No Tension MD free Start, Before Heat Set 1.53961.5398 1.5398 1.5396 0.0009 0.1590 After 1st Heat Set 1.5447 1.54371.5461 1.5443 0.0006 0.1542 Diff 1st-Start 0.0047 −0.0003 −0.0048

The first two PET examples demonstrate the use of heat setting with asmall continuing stretch ending at an addition 5% draw ratio. In theseexamples, the films were mounted taut in both x(TD) and y(MD)directions. Due to the discontinuous nature of the edge gripping system,the MD constraint is less than constant initial strain. The filmsdemonstrate an increased nx and nearly constant ny and nz. A very smallincrease in asymmetry is noted. Removal of the MD constraint may reducethis asymmetry.

In the third PET example, the films were mounted taut in x and y but nostretching took place during heat setting. Again, the x index (TD index)increased while the ny held nearly constant. Surprisingly, the nzincreased upon heat setting, although some of this effect may be aresult of the measurement as the knee sharpened after heat setting. Thusthe asymmetry may have decreased or at least maintained in this case.

In the fourth PET example, the film was mount taunt only in the xdirection. The largest increase in nx was observed here. The ny and nzeach increased marginally.

In the fifth PET example, the film was begun with a slight slack builtinto the mounting. The 2.5 inch TD span was deflected about 0.25 inchout-of-plane by this slack. In this case, the nx increase was justslightly one-half that of the fourth PET example, but the ny increasewas nearly double. The nz only marginally increased. The film appearedtaut at the end of the heat setting.

In the sixth and seventh PET examples, the films were provided withdouble the initial slack of the fifth PET example. In these replicatecases, the films retained a slight residual slack after heat setting.The nx essentially remained constant in these cases, even as the ny andnz increased in nearly identical amounts. In the sixth case, the filmwas measured after the first heat setting and re-mounted for a secondstep, again 3 minutes at 175° C. Further increases in nx and ny wereobserved, again at nearly constant nx.

The effect of heat setting on the level of crystallinity was estimatedusing the estimated increase in density as inferred by the increases inthe indices of refraction in accord with an anisotropic analogue of theLorenz-Lorentz relationship as described in U.S. Pat. No. 6,788,463,incorporated herein by reference (see Lorentzian in Tables 2 and 4). Theamorphous density was taken as 1.335 g/cc and the fully crystallinedensity as 1.457 g/cc. The volumetric polarizability was taken as0.73757 cc/g. As shown in Table 2, the analysis indicates that thecrystallinity (e.g. a crystal fraction of 0.32 fraction equals a 32%crystallinity) increased from just over 30% in these samples to 40% inthe case of the double treated sixth PET example. In an exemplaryembodiment, the PET has a crystallinity after heat setting greater than33% (e.g., Example 2); in another exemplary embodiment, the PET has acrystallinity greater than 36% (e.g., Example 3 and Example 6 after1^(st) heat set); in another exemplary embodiment, the PET has acrystallinity greater than 37% (e.g., Example 1 and Example 7 after1^(st) heat set); in another exemplary embodiment, the PET has acrystallinity greater than 38% (e.g., Example 5); in another exemplaryembodiment, the PET has a crystallinity greater than 39% (e.g., Example4); and in another exemplary embodiment, the PET has a crystallinitygreater than 40% (e.g., Example 6 after 2^(nd) heat set).

Higher extremes in time and temperature would be expected to furtherincrease the levels of crystallinity and index changes.

TABLE 2 TD MD ND Relative Density Crystal PET Examples ave ave aveBirefringence Lorentzian est est. Example 1: Slight Draw TD w heatStart, Before Heat Set 1.6987 1.5405 1.5389 0.0099 1.013126 1.37360040.316 5% stretch& Heat Set 1.7074 1.5435 1.5389 0.0280 1.0182891.3805995 0.374 Diff 1st-Start 0.0087 0.0031 0.0000 0.057 Example 2:Slight Draw TD w heat Start, Before Heat Set 1.6982 1.5403 1.5387 0.01021.012722 1.3730524 0.312 5% stretch& Heat Set 1.7038 1.5403 1.53770.0161 1.014613 1.3756157 0.333 Diff 1st-Start 0.0056 0.0000 −0.00100.021 Example 3: Constrain TD/MD Start, Before Heat Set 1.7003 1.54131.5376 0.0227 1.013557 1.3741843 0.321 After 1st Heat Set 1.7045 1.54251.5416 0.0058 1.017881 1.380046 0.369 Diff 1st-Start 0.0042 0.00120.0039 0.048 Example 4: Taut start MD free Start, Before Heat Set 1.69741.5413 1.5394 0.0123 1.013202 1.3737032 0.317 After 1st Heat Set 1.70811.5431 1.5421 0.0059 1.019906 1.3827922 0.392 Diff 1st-Start 0.01070.0017 0.0027 0.075 Example 5: Slight Slack at start TD Tension dev. MDfree Start, Before Heat Set 1.6980 1.5408 1.5396 0.0074 1.0133111.3738507 0.318 After 1st Heat Set 1.7040 1.5439 1.5429 0.0061 1.0189371.3814789 0.381 Diff End-Start 0.0059 0.0031 0.0033 0.063 Example 6: TDno tension MD free Start, Before Heat Set 1.6978 1.5418 1.5394 0.01561.013603 1.3742459 0.322 After 1st Heat Set 1.6962 1.5465 1.5441 0.01591.017499 1.3795283 0.365 Diff 1st-Start −0.0016 0.0047 0.0048 0.043After 2nd Heat Set 1.6979 1.5483 1.5481 0.0015 1.021004 1.3842807 0.404Diff 2nd-start 0.0002 0.0065 0.0087 0.082 Example 7: No Tension MD freeStart, Before Heat Set 1.6995 1.5405 1.5396 0.0056 1.013843 1.37457170.324 After 1st Heat Set 1.6991 1.5449 1.5443 0.0039 1.018043 1.3802660.371 Diff 1st-Start −0.0004 0.0044 0.0047

PEN Examples:

Multilayer optical films (MOF) with PEN skin layers were highly extendedusing the parabolic tentering process. Film was used from a single MDlane of the continuous final film to enhance reproducibility of theinitial state. The index development in the PEN skin layers after thestretching step was measured using an average of “top” and “bottom”sides using a Metricon Prism coupler. The method of index measurementsusing the leading edge of the wave modes and intensity knees, as per thePET examples, were again used.

Two PEN skin replicate examples were made (Examples 8-9). The films werebegun with a slight slack built into the mounting. The 2.5 inch TD spanwas deflected about 0.5 inch out-of-plane by this slack. Heat settingconditions were applied for 3 minutes at 175° C. The film retainedresidual slack after treatment. The index changes with heat setting arepresented in Table 3.

As indicated in Table 3 below, the leading edge method (“by knee”)compared very closely to the “offset” mode method provided in thesoftware accompanying the Metricon. (The leading edge method wasoperator estimated, rather than using the knee estimation softwareaccompanying the Metricon.) Using these methods the initial indicesaveraged 1.868, 1.569 and 1.553 at 632.8 nm, for nx, ny and nzrespectively. The initial relative birefringence using these indexvalues is thus 0.053.

TABLE 3 In-plane Refractive orientation/ Indices at TM mode Uniax 632.8nm measured limit TD TD TD MD MD MD TD/z MD/z TD/z MD/z ND MD − TD − PENExamples top bot ave top bot ave top top bot bot ave Z MD Example 8:Meas. By Knee, 1.8690 1.8662 1.8676 1.5681 1.5699 1.5690 1.5526 1.55421.5514 1.5527 0.0163 0.2986 Start before Heat Set Meas. By Offset,1.8671 1.8677 1.8674 1.5677 1.5691 1.5684 1.5530 1.5523 1.5511 1.55261.5523 0.0162 0.2990 Start before Heat Set Averages between 1.86751.5687 1.5525 0.2988 measurement methods After 1st Heat 1.8593 1.86021.8598 1.5702 1.5709 1.5706 1.5577 1.5582 1.5580 1.5577 1.5579 0.01260.2892 Set Diff 1st-Start −0.0078 0.0015 0.0052 −0.0036 −0.0096 Example9: Meas. By Knee 1.8671 1.8672 1.8672 1.5699 1.5704 1.5702 1.5506 1.55121.5496 1.5523 1.5509 0.0192 0.2970 After 1st Heat 1.8588 1.8587 1.85881.5740 1.5706 1.5723 1.5609 1.5598 1.5613 1.5607 0.0116 0.2865 Set Diff1st-Start −0.0084 0.0022 very poor 0.0097 −0.0076 −0.0106 knees hereAfter 2nd Heat 1.8577 1.8577 1.5825 1.5825 1.5738 1.5754 1.5746 0.00790.2752 Set Diff 2nd-start −0.0095 0.0124 0.0237 −0.0113 −0.0218

As seen in the PET low/no tension cases, the ny and nz increased.However, under these conditions, the nx actually dropped significantly.One would therefore expect less subsequent film shrinkage. Onedifference between these cases is that the current PEN case does nothave a large toe-in condition after stretch that the PET cases have.Thus, some of this index drop is related to residual stress relief andvisco-elastic relaxation during the heat setting. It is expected thatthe PEN skin films after heat setting have significantly less hightemperature shrinkage (for example, at temperatures above the glasstransition of the PEN, up to the heat set temperature) than theuntreated initial film. In the second replicate, the film was measuredafter the first heat setting and re-mounted for a second step, this timefor 3 minutes at 190° C. Significant increases in ny and nz wereobserved, with only a slight drop in nx, in agreement with the trend ofthe sixth and seventh PET skin cases.

The untreated PEN-skin film, the film after the first heat set and thefilm after the more severe second heat set were all tested formechanical dent/imprint resistance. To test for imprint resistance, apressure sensitive adhesive was laminated onto one surface of the filmand that surface was then laminated onto a glass slide. A piece of BEF™brightness enhancement film, available from 3M Company, St. Paul Minn.was placed with its micro-textured surface against an exposed filmsurface with a weight of 150 g on top to ensure intimate contact. Theresulting pressure was estimated as 200 g/sq. inch. The film was thentested for 24 hours at 85° C. The initial film and the film with thelight first heat setting were modestly imprint resistance, butsignificant denting occurred. The film with the second, more severe heatsetting exhibited almost no dents.

Again, the effect of heat setting on the level of crystallinity wasestimated using the estimated increases in density as inferred by theincreases in the indices of refraction in accord with an anisotropicanalogue of the Lorenz-Lorentz relationship as described in U.S. Pat.No. 6,788,463. The amorphous density for PEN was taken as 1.329 g/cc andthe fully crystalline density as 1.407 g/cc. The volumetricpolarizability was taken as 0.81501 cc/g. As shown in Table 4, theanalysis indicates that the crystallinity increased very little in thefirst heat step, providing further evidence for the mechanism of indexchanges as one of visco-elastic relaxation.

This also indicates that the level of crystallinity is a major factor inimprint resistance. After the more extreme heat setting, the filmacquired a much higher level of crystallinity, estimated at about 48%.This more crystalline final film exhibited the highest level of imprintresistance among the example presented here.

In an exemplary embodiment, the PEN has a crystallinity after heatsetting greater than 28% (e.g., Example 8); in another exemplaryembodiment, the PEN has a crystallinity greater than 30% (e.g., Example9 after 1^(st) heat set); in another exemplary embodiment, the PEN has acrystallinity greater than 48% (e.g., Example 9 after 2^(11d) heat set).

As shown by the examples, second or subsequent heat setting steps can beused to obtain desired film properties.

TABLE 4 TD MD ND Relative Density Crystal PEN Examples ave ave aveBirefringence Lorentzian est est. Example 8: Meas. By Knee, Start 1.86761.5690 1.5527 0.0530 1.100845 1.3507131 0.278 before Heat Set Meas. ByOffset, Start 1.8674 1.5684 1.5523 0.0526 1.100255 1.3499895 0.269before Heat Set Averages between 1.8675 1.5687 1.5525 0.0528 0.274measurement methods After 1st Heat Set 1.8598 1.5706 1.5579 0.04281.101116 1.3510461 0.282 Diff 1st-Start −0.0078 0.0015 0.0052 0.009Example 9: Meas. By Knee 1.8672 1.5702 1.5509 0.0627 1.100354 1.35011140.271 After 1st Heat Set 1.8588 1.5723 1.5607 0.0398 1.102886 1.35321720.310 Diff 1st-Start −0.0084 0.0022 0.0097 0.040 After 2nd Heat Set1.8577 1.5825 1.5746 0.0283 1.11388 1.3667074 0.483 Diff 2nd-start−0.0095 0.0124 0.0237 0.213

CoPEN Examples: Co-Polyester Example 10:

A co-polyester, intermediate in composition between PEN and PET, wasformed by charging an extruder with a pellet mixture of 85 mol % PEN(with an approximate intrinsic viscosity (IV) of 0.5) and 15 mol % PET(with an approximate IV of 0.8). These transesterified in-situ duringextrusion and were cast to form a clear unoriented cast web comprising aso-called 85/15 coPEN. A film comprising this material can be used as abirefringent layer in a multilayer optical film, e.g. a reflectivepolarizer film.

A strip 6 cm long by 2.5 cm wide was cut from the cast web and drawn ona laboratory stretching apparatus. The strip was pre-heated for 1 minuteat 130 degrees C. and drawn along its length without constraint in itswidth at a nominal draw rate of 20%/second to a final draw ratio of 5.5as measured by fiducial marks placed on the film before stretching.

After stretching, the film was quenched to room temperature and therefractive indices were measured at 632.8 nm using the Metricon PrismCoupler as 1.8436, 1.5668 and 1.5595 along the length, width andthickness directions, respectively. Thus, a relative birefringence of0.061 was obtained after stretching.

The oriented film was then mounted with slight initial tension along thelength, and uncontrained in the width, and heat at 170 degrees C. fortwo minutes. The film was quenched again, and the refractive indiceswere measured at 632.8 nm using the Metricon Prism Coupler as 1.8404,1.5718 and 1.5492 along the length, width and thickness directions,respectively. Thus, a relative birefringence of 0.081 was obtained afterstretching.

Co-Polyester Example 11:

An 85/15 coPEN was formed and drawn in similar manner to of example 10.A film comprising this material can be used as a birefringent layer in amultilayer optical film, e.g. a reflective polarizer film.

The oriented film was then mounted with slight initial tension along thelength, and uncontrained in the width, and heated at 190 degrees C. for30 seconds. The film was further heated for 90 more seconds at 190degrees C. while the draw ratio was reduced from its initial 5.5× afterstretching to 4.7× after heat setting. The film was quenched again, andthe refractive indices were measured at 632.8 nm using the MetriconPrism Coupler as 1.8185, 1.5827 and 1.5576 along the length, width andthickness directions, respectively. Thus, a relative birefringence of0.101 was obtained after stretching.

Heat Set Uniaxially Oriented Multi-Layer Optical Films withPolycarbonate/Copolyester Blend Isotropic Layers

Comparative Example 1 Multilayer OpticalFilm—PEN/CoPEN5545HD/CoPEN7525HD Reflective Polarizer

A multilayer reflective polarizer film was constructed with firstoptical layers created from a polyethylene naphthalate and secondoptical layers created from copolyethylenenaphthalate (CoPEN5545HD) andskin layers or non-optical layers created from a higher Tgcopolyethylenenaphthalate (CoPEN7525HD).

The above described PEN and CoPEN5545HD were coextruded through amultilayer melt manifold to create a multilayer optical film with 275alternating first and second optical layers. This 275 layer multi-layerstack was divided into 3 parts and stacked to form 825 layers. The PENlayers were the first optical layers and the CoPEN5545HD layers were thesecond optical layers. In addition to the first and second opticallayers, a set of non-optical layers, also comprised of CoPEN5545HD werecoextruded as PBL (protective boundary layers) on either side of theoptical layer stack. Two sets of skin layers comprising CoPEN7525HD werealso coextruded on the outer side of the PBL non-optical layers throughadditional melt ports. The multi-layer film construction was in order oflayers: CoPEN7525HD skin layer, a CoPen5545HD PBL, 825 alternatinglayers of optical layers PEN/CoPEN5545HD, a second CoPen5545HD PBL, anda second skin layer CoPEN7525HD.

The multilayer extruded film was cast onto a chill roll at 15 meters perminute (45 feet per minute) and heated in an oven at 150° C. (302° F.)for 30 seconds, and then uniaxially oriented at a 5.5:1 draw ratio.After orientation, the drawn multi-layer film was passed through a heatset oven at 200° C. for 15 seconds. A reflective polarizer film ofapproximately 150 microns (6 mils) thickness was produced which was toomechanically brittle for web handling, winding into a roll, ordie-cutting into film parts without breaking.

Comparative Example 2 Multilayer OpticalFilm—CoPEN9010/CoPEN-Tbia/CoPEN-Tbia Reflective Polarizer

A multilayer reflective polarizer film was constructed with firstoptical layers created from a copolyethylenenaphthalate (CoPEN9010) andsecond optical layers created from copolyethylenenaphthalate(CoPEN-tbia) and skin layers or non-optical layers created fromcopolyethylenenaphthalate (CoPEN-tbia).

The above described CoPEN9010 and CoPEN-tbia were coextruded through amultilayer melt manifold to create a multilayer optical film with 275alternating first and second optical layers. The CoPEN9010 layers werethe first optical layers and the CoPEN-tbia layers were the secondoptical layers. In addition to the first and second optical layers, aset of non-optical layers, also comprised of CoPEN-tbia, were coextrudedas PBL (protective boundary layers) on either side of the optical layerstack. Two sets of skin layers comprising CoPEN-tbia were alsocoextruded on the outer side of the PBL non-optical layers throughadditional melt ports. The multi-layer film construction was in order oflayers: CoPEN-tbia skin and PBL layers, 275 alternating layers ofoptical layers CoPEN9010/CoPEN-tbia, and a second set of skin and PBLlayers of CoPEN-tbia.

The multilayer extruded film was cast onto a chill roll at 15 meters perminute (45 feet per minute) and heated in an oven at 150° C. (302° F.)for 30 seconds, and then uniaxially oriented at a 6.5:1 draw ratio.After orientation, the drawn multi-layer film was passed through a heatset oven at 200° C. for 15 seconds. A reflective polarizer film ofapproximately 37 microns (1.5 mils) thickness was produced which was toomechanically brittle for web handling, winding into a roll, ordie-cutting into film parts without breaking.

Example 3 Multilayer Optical Film—PEN/CoPEN5050HH/SA115 ReflectivePolarizer Film

A multilayer reflective polarizer film was constructed with firstoptical layers created from a polyethylene naphthalate and secondoptical layers created from copolyethylene naphthalate (CoPEN5050HH) andskin layers or non-optical layers created from a cycloaliphaticpolyester/polycarbonate blend commercially available from EastmanChemical CO. under the tradename “SA115.”

The above described PEN and CoPEN5050HH were coextruded through amultilayer melt manifold to create a multilayer optical film with 275alternating first and second optical layers. This 275 layer multi-layerstack was divided into 3 parts and stacked to form 825 layers. The PENlayers were the first optical layers and the CoPEN5050HH layers were thesecond optical layers. In addition to the first and second opticallayers, a set of non-optical layers, also comprised of CoPEN5050HH werecoextruded as PBL (protective boundary layers) on either side of theoptical layer stack. Two sets of SA115 skin layers were also coextrudedon the outer side of the PBL non-optical layers through additional meltports. The construction was in order of layers: SA115 skin layer,CoPEN5050HH PBL layer, 825 alternating layers of optical layers of PENand CoPEN-5050HH, a second CoPEN5050HH PBL layer, and a second SA115skin layer.

The multilayer extruded film was cast onto a chill roll at 15 meters perminute (45 feet per minute) and heated in an oven at 150° C. (302° F.)for 30 seconds, and then uniaxially oriented at a 5.5:1 draw ratio.After orientation, the drawn multi-layer film was passed through a heatset oven at 200° C. for 15 seconds. A reflective polarizer film ofapproximately 150 microns (6 mils) thickness was produced. This film wasnot mechanically brittle, could easily be wound into a film roll, anddie cut into film parts without breaking.

Example 4 Multilayer Optical Film—CoPEN9010/SA115/SA115 ReflectivePolarizer Film

A multilayer reflective polarizer film was constructed with firstoptical layers created from a polyethylene naphthalate (CoPEN9010), andwith second optical layers and skin layers created from a cycloaliphaticpolyester/polycarbonate blend commercially available from EastmanChemical under the series tradename “SA115”.

This CoPEN9010 was coextruded with SA115 through a multilayer meltmanifold to create a multilayer optical film with 275 alternating firstand second optical layers. The CoPEN9010 layers were the first opticallayers and the SA115 layers were the second optical layers. In additionto the first and second optical layers, a set of non-optical layers,also comprised of SA115, were coextruded as PBL (protective boundarylayers) on either side of the optical layer stack. Two skin layerscomprised of SA115 were also coextruded on the outer side of the PBLnon-optical layers through additional melt ports. The construction wasin order of layers: SA115 outer skin and PBL layers, 275 alternatingoptical layers of CoPEN9010 and SA115, and a further set of SA115 PBLand outer skin layers.

The multilayer extruded film was cast onto a chill roll at 22 meters perminute (66 feet per minute) and heated in an oven at 139° C. (283° F.)for 30 seconds, and then nearly truly uniaxially oriented at a 6:1 drawratio. After orientation, the drawn multi-layer film was passed througha heat set oven at 200° C. for 15 seconds. A reflective polarizer filmof approximately 30 microns (1.2 mils) was produced which was notmechanically brittle, could easily be wound into a film roll, and diecut into film parts without breaking.

Description of Polymer Making for Above Examples. Manufacture of PEN

The polyethylene naphthalate (PEN) used to form the first optical layerswas synthesized in a batch reactor with the following raw materialcharge: dimethyl naphthalene dicarboxylate (136 kg), ethylene glycol (73kg), manganese (II) acetate (27 g), cobalt (II) acetate (27 g) andantimony (III) acetate (48 g). Under a pressure of 2 atmospheres (1520torr or 2×105 N/m2), this mixture was heated to 254° C. while removingmethanol (a transesterification reaction by-product). After 35 kg ofmethanol was removed, triethyl phosphonoacetate (49 g) was charged tothe reactor and the pressure was gradually reduced to 1 ton (131 N/m2)while heating to 290° C. The condensation reaction by-product, ethyleneglycol, was continuously removed until a polymer with an intrinsicviscosity of 0.48 dL/g (as measured in 60/40 wt. %phenol/o-dichlorobenzene) was produced.

Manufacture of CoPEN9010

The copolyethylene naphthalate (CoPEN9010) used to form the firstoptical layers was synthesized in a batch reactor with the following rawmaterial charge: 126 kg dimethyl naphthalene dicarboxylate, 11 kgdimethyl terephthalate, 75 kg ethylene glycol, 27 g manganese acetate,27 g cobalt acetate, and 48 g antimony triacetate. Under pressure of 2atm (2×105 N/m2), this mixture was heated to 254° C. while removingmethanol. After 36 kg of methanol is removed, 49 g of triethylphosphonoacetate was charged to the reactor and than the pressure wasgradually reduced to 1 ton while heating to 290° C. The condensationreaction by-product, ethylene glycol, was continuously removed until apolymer with an intrinsic viscosity of 0.50 dL/g, as measured in 60/40wt. % phenol/o-dichlorobenzene, was produced.

Manufacture of CoPEN5545HD

The copolyethylene-hexamethylene naphthalate polymer (CoPEN5545HD) usedto form the second optical layers was synthesized in a batch reactorwith the following raw material charge: dimethyl2,6-naphthalenedicarboxylate (88.5 kg), dimethyl terephthalate (57.5kg), 1, 6-hexane diol (4.7 kg), ethylene glycol (81 kg), trimethylolpropane (239 g), cobalt (II) acetate (15 g), zinc acetate (22 g), andantimony (III) acetate (51 g). The mixture was heated to a temperatureof 254° C. at a pressure of two atmospheres (2×105 N/m2) and the mixtureallowed to react while removing the methanol reaction product. Aftercompleting the reaction and removing the methanol (approximately 39.6kg) the reaction vessel was charged with triethyl phosphonoacetate (37g) and the pressure was reduced to one ton (263 N/m2) while heating to290° C. The condensation by-product, ethylene glycol, was continuouslyremoved until a polymer with intrinsic viscosity 0.56 dl/g as measuredin a 60/40 weight percent mixture of phenol and o-dichlorobenzene wasproduced. The CoPEN5545HD polymer produced by this method had a glasstransition temperature (Tg) of 94° C. as measured by differentialscanning calorimetry at a temperature ramp rate of 20° C. per minute.The CoPEN5050HH polymer had a refractive index of 1.612 at 632 nm.

Manufacture of CoPEN7525HD

The copolyethylene-hexamethylene naphthalate polymer (CoPEN7525HD) usedto form the second optical layers was synthesized in a batch reactorwith the following raw material charge: dimethyl2,6-naphthalenedicarboxylate (114.8 kg), dimethyl terephthalate (30.4kg), 1, 6-hexane diol (5.9 kg), ethylene glycol (75 kg), trimethylolpropane (200 g), cobalt (II) acetate (15 g), zinc acetate (22 g), andantimony (III) acetate (51 g). The mixture was heated to a temperatureof 254° C. at a pressure of two atmospheres (2×105 N/m2) and the mixtureallowed to react while removing the methanol reaction product. Aftercompleting the reaction and removing the methanol (approximately 39.6kg) the reaction vessel was charged with triethyl phosphonoacetate (37g) and the pressure was reduced to one ton (263 N/m2) while heating to290° C. The condensation by-product, ethylene glycol, was continuouslyremoved until a polymer with intrinsic viscosity 0.52 dl/g as measuredin a 60/40 weight percent mixture of phenol and o-dichlorobenzene wasproduced. The CoPEN7525HD polymer produced by this method had a glasstransition temperature (Tg) of 102° C. as measured by differentialscanning calorimetry at a temperature ramp rate of 20° C. per minute.The CoPEN7525HD polymer had a refractive index of 1.624 at 632 nm.

Manufacture of CoPEN5050HH

The copolyethylene-hexamethylene naphthalate polymer (CoPEN5050HH) usedto form the second optical layers was synthesized in a batch reactorwith the following raw material charge: dimethyl2,6-naphthalenedicarboxylate (80.9 kg), dimethyl terephthalate (64.1kg), 1, 6-hexane diol (15.45 kg), ethylene glycol (75.4 kg), trimethylolpropane (2 kg), cobalt (II) acetate (25 g), zinc acetate (40 g), andantimony (III) acetate (60 g). The mixture was heated to a temperatureof 254° C. at a pressure of two atmospheres (2×105 N/m2) and the mixtureallowed to react while removing the methanol reaction product. Aftercompleting the reaction and removing the methanol (approximately 42.4kg), the reaction vessel was charged with triethyl phosphonoacetate (55g) and the pressure was reduced to one ton (263 N/m2) while heating to290° C. The condensation by-product, ethylene glycol, was continuouslyremoved until a polymer with intrinsic viscosity 0.55 dl/g as measuredin a 60/40 weight percent mixture of phenol and o-dichlorobenzene wasproduced. The CoPEN5050HH polymer produced by this method had a glasstransition temperature (Tg) of 85° C. as measured by differentialscanning calorimetry at a temperature ramp rate of 20° C. per minute.The CoPEN5050HH polymer had a refractive index of 1.601 at 632 nm.

Manufacture of CoPEN-Tbia

The synthesis of the CoPEN-tbia polymer was carried out in a batchreactor which was charged with the following materials: dimethyl2,6-naphthalenedicarboxylate (47.3 kg), dimethyl terephthalate (18.6kg), 1,4-cyclohexane dimethanol (40.5 kg), neopentyl glycol (15 kg),ethylene glycol (41.8 kg), trimethylol propane (2 kg), cobalt (II)acetate (36.3 g), zinc acetate (50 g), and antimony (III) acetate (65g). The mixture was heated to a temperature of 254° C. at a pressure oftwo atmospheres (2×105 N/m2) and the mixture allowed to react whileremoving the methanol reaction product.

After completing the reaction and removing all of the methanol(approximately 13.1 kg) the reaction vessel was charged withtertiary-butyl isophthalate (43.2 kg). The reaction was continued at254° C. until approximately 7.4 kg of water was removed and the reactionwas complete. The reaction vessel was charged with triethylphosphonoacetate (70 g) and the pressure was reduced to one ton (263N/m2) while heating to 290° C. The condensation by-product, ethyleneglycol, was continuously removed until a polymer with intrinsicviscosity 0.632 dl/g as measured in a 60/40 weight percent mixture ofphenol and o-dichlorobenzene was produced. The CoPEN-tbia polymerproduced by this method had a glass transition temperature (Tg) of 102°C. as measured by differential scanning calorimetry at a temperatureramp rate of 20° C. per minute. The CoPEN-tbia polymer had a refractiveindex of 1.567.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

1. A multilayer reflective polarizer comprising a plurality ofpolyester-based polymeric layers, the film defining in-plane orthogonalx- and y-directions and a z-direction orthogonal to the x- andy-directions, the reflective polarizer stretched along the x-direction,each polyester-based polymeric layer having refractive indices nx, nyand nz along the respective x-, y- and z-directions, the reflectivepolarizer heat set at a temperature above a glass transition temperatureand below a melting point of each of the polyester-based polymericlayers, wherein for at least one polyester-based polymeric layer, theheat setting results in a decrease in nx and |ny−nz|/|nx−nu|, and anincrease in ny and nz, wherein nu is an average of ny and nz.